Last week, one of the things that struck me about the coverage of the two main tech conferences devoted to start-ups, DEMOfall and TechCrunch50, was the almost complete lack of discussion–or, more appropriately, worry–about the troubled economy.

This, even though the subprime mortgage crisis remains in full swing, along with the continuing turmoil around the stability of Wall Street financial firms, as well as Fannie Mae and Freddie Mac. Also, let’s not forget those sky-high gas prices.

Still, one entrepreneur after another got up and spun stories of companies built on fancy new ad models to come or the more hidden premise that they would get bought by a bigger fish up the food chain like Google (GOOG) or Microsoft (MSFT).

But as I wrote last week, all those stocks are suffering, and those companies are likely to be retrenching rather than expanding.

In fact, at an interview with Internet entrepreneur and investor Peter Thiel at TC50, he called Web companies undervalued. (See my video interview with Thiel here last November, in which he says a lot of the same stuff.)

As he told me almost a year ago, Thiel repeated his contention that the technology sector was still not in a bubble.

Well, perhaps not, but that’s actually because most companies in Web 2.0–despite their massive valuations over the last few years–aren’t going to have the chance get frothy and light enough to become so poppable.

Instead, most will likely fizzle away quietly, with no exits in sight as the economy weakens and puts a vise grip on companies that cannot survive the very tough financial road ahead.

That means the popular Web 2.0 maxim of “growth before profits” is in for a very bumpy ride.

It’s going to be especially true after this weekend’s latest round of bad news–the bankruptcy of Lehman Brothers, the sale of Merrill Lynch, the impending crisis at banks like Washington Mutual.

And with more to come, I could not help but remember the well-known political phrase used in the 1992 presidential race by the Clinton team: It’s the economy, stupid.

And that’s true even here in Silicon Valley, where the sun always seems to shine, even as the clouds gather.

(Also, here is a very good piece on the topic by CNET’s Larry Dignan on some more details of the impact on tech firms, specifically on selling products to financial firms.)

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

Here’s part three of the conference’s most popular demo, by well-known inventor Dean Kamen, who showed off his latest creation, the “Luke Arm” prosthesis.

The state-of-the-art bionic arm looks and feels to its user like his or her native limb.

The video of the demo is in three parts, all of which have been posted today.

In this segment, the DEKA Research founder and Segway creator talks about the FIRST Robotics competition, which Kamen founded to interest young people in science and technology.

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

Here’s part two of the conference’s most popular demo, with well-known inventor Dean Kamen, who showed off his latest creation, the “Luke Arm” prosthesis.

The state-of-the-art bionic arm looks and feels to its user like his or her native limb.

The video of the demo is in three parts, all of which I will post all today.

In this video, the DEKA Research founder and Segway creator talks more about how the arm’s control was designed and how it was received by a group of injured veterans.

Kamen also talks about the FIRST Robotics competition, which he started to get young people interested in science and technology.

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

Here’s part one of the conference’s most popular demo, with well-known inventor Dean Kamen (pictured here), who showed off his latest creation, the “Luke Arm” prosthesis.

The state-of-the-art bionic arm looks and feels to its user like his or her native limb.

The video of the demo is in three parts, all of which I will post today.

In this video, the DEKA Research founder and Segway creator talks about how he began the project after a visit to his lab by Defense Department officials, who told him that while battleground deaths were down, severe limb injuries were up.

Within a year, the prosthesis was built. In the video, Kamen shows the arm in use, including a man who is able to feed himself after more than a dozen years of being fed by someone else. Kamen also talks about how the arm can be controlled.

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

Next up is evri, the semantic Web guide start-up, which is backed by billionaire techie Paul Allen.

There are two videos showing the whole demo below.

In the first one, evri Founder Neil Roseman talks about the application that graphs the Web via grammatical information. He uses it to discover info about and people related to Sen. Barack Obama, for example, which is done in a “horizontal” manner. He somehow ends up at “Sex and the City” star Sarah Jessica Parker.

In the second, Roseman finds content about Walt Mossberg and me, as well as celebrity Rachael Ray, and talks about the business plan and inspiration for evri.

Walt Mossberg and BoomTown are in San Diego for DEMOfall, which is one of two big tech start-up demo conferences taking place this week (the other is TechCrunch50 in San Francisco).

At both conferences, a passel of start-ups come to show off their wares to an audience of media, venture capitalists and one other.

The big focus at DEMOfall this year has been on social networking, mobile technology and cloud computing–three of the big Web 2.0 trends–although I liked the company that can make a stolen cellphone sound like a really annoying car alarm.

Walt and I also appeared here this afternoon onstage in what was called “Head-to-Head,” a feature we hope to debut on this site soon.

Essentially, the idea is to discuss and debate the top tech issues of the moment, although it was not meant to be a wrestling match between Walt and me (perish the thought!).

Thus, we interviewed each other–much as we do tech leaders on the stage of our D: All Things Digital conference.

Yesterday’s topics included the Google new Chrome browser, the growing power of the search giant, business prospects for social networking and the future of cellphones.

(I also did a decent impression of Alaska Governor and Republican Vice Presidential candidate Sarah Palin, but that’s another story.)

It will all eventually be available on DEMOfall’s Web site soon, but here’s my video about the conference so far:

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

Next up is the demo for 23andMe, the Web-based, direct-to-consumer genome site that showed off its new 23andWe service at the conference.

The new offering is aimed at conducting large-scale genome-wide association studies with the help of individuals willing to share information about their health and other personal traits.

(Today, 23andMe announced that it was dropping the price of its service from $1,000 to $399 and unveiling new technology enhancements and community features, as well as striking a partnership with Ancestry.com.)

There are two videos showing the whole demo below.

In the first one, 23andMe Co-Founders Linda Avey and Anne Wojcicki introduce 23andWe, explain the main service and ask News Corp. head Rupert Murdoch, Walt and me about our tolerance for milk and about our racing abilities (News Corp. is the owner of Dow Jones and this Web site).

In the second one, Avey and Wojcicki survey Murdoch on his genetic traits and show me what genes my kids have in common (and discover that I am not hyperactive).

But–as many readers have requested–they will all be available in their entirety in this column.

In the less contentious spirit of DEMOfall and TechCrunch50, two demo conferences taking place simultaneously this week, we’re happy to bring you all the demos we had onstage at D6.

First up is TransMedia’s Glide, an operating system that cuts across all operating systems.

There are two videos showing the whole demo below.

In the first one, TransMedia Chairman and CEO Donald Leka walks Walt and Kara through Glide on Mac OSX, Windows and the iPhone.

In the second, Leka demos the engaging kids’ version of Glide, shows how the OS runs in a browser on an iPhone and talks about the company’s business model.