“Demo Day” will be held for the first time as one grand event at the Computer History Museum, after the end-of-program affair had swelled into multiple sessions over multiple days as it outgrew the orange-walled Y Combinator offices time and again.

“YC” is the influential start-up accelerator that has produced hits like Dropbox and Airbnb and also helped reshape the start-up world around the ideal of its young, hacker-led teams. Over the years, it has become a sort of entrepreneurial grad school; in fact, YC recently announced it will start accepting applications from groups who don’t yet have a company idea.

Some 66 start-ups participated in the current session, and more than 500 angel and venture investors are scheduled to attend.

Y Combinator created a tablet-optimized portal for investors and start-ups to connect with each other before the day ends. To give you an idea of the anticipated feeding frenzy, here’s what the firm told investors: “We recommend you sign up for the online system, because the increased opportunities to meet at Demo Day may mean that some start-ups will have handshake deals for as much money as they want by the end of Demo Day itself.”

This year’s batch of companies have each already had the opportunity to raise some $168,000 while giving up a small bit of equity, and some start-ups have already launched to the public.

The group includes one founder who is on his third round at Y Combinator. That’s Justin Kan of Exec, which he called “an Uber for TaskRabbit” when we interviewed him before it launched. Kan was formerly founder of Kiko and Justin.tv, and is also a part-time partner at Y Combinator.

(Image courtesy of Flickr user brianjmatis)

The service uses Justin.tv’s mobile live-streaming infrastructure, so videos are uploaded as much as possible while users are still taking them. If users don’t have Internet access, the video will upload when it can and then send a push notification. But there’s nothing live about Socialcam.

The intent, instead, is to encourage mobile video-sharing the way apps like Instagram encourage photo-sharing. The photo-sharing app Path does offer video uploads, but it limits them to 10 seconds, and even so I’ve had uploads fail. Justin.tv has four years’ worth of experience with video uploads, so it hopes it can get the job done.

The Socialcam app, which obviously has different branding than Justin.tv, came out of Justin.tv’s live mobile broadcasting app, which launched eight months ago and has been downloaded four million times. The company was getting 20 to 30 percent of broadcasts from mobile and finding that mobile broadcasters were far more likely to share their clips on Facebook and Twitter, said Justin.tv namesake and President Justin Kan. But 90 percent of the views of mobile videos were after the fact. It just wasn’t that important that mobile videos be watchable live.

Socialcam asks users to log into Facebook and invites them to share videos via Facebook, Twitter, email and SMS. Videos are public by default, but there’s no directory to find them, so viewers will need to have the URL or see them shared somewhere. Users can also see a stream of their Socialcam friends’ videos within the app, and like and share them directly.

When Facebook friends are included and tagged in Socialcam videos, the videos are automatically posted to their Facebook walls, just like that service’s native photo-tagging feature. In fact, Socialcam seems similar to what Facebook might launch if it ever put any development resources into video, or if YouTube put more focus on personal and social video, but that hasn’t happened yet.

Another interesting approach to mobile video sharing I saw recently is HighlightCam, which is sort of like Animoto for iPhone videos–automatically remixing videos into a montage of the most interesting segments. Socialcam, on the other hand, doesn’t do video editing or filters. “Filters don’t make videos interesting,” was the justification Kan gave me.

It’s not clear that people need yet another app just for mobile video uploads, but at least Socialcam addresses a real issue: It’s hard to get videos off of phones without using a cord. Whether the company can create its own mobile video community is another question.

In her post late last week that I missed (I was traveling and therefore not aware of her cruel, cruel taunts), she pointed with dread to a recent promise I made to try out a new wearable camera I saw at the TechCrunch40 conference earlier in the week, a la Justin Kan of Justin.tv.

Writes McCarthy:

One thing to keep in mind about this new development: Swisher, unlike Kan, is tiny–about 5’2″, if that. Any hat-camera she wears will be eye-level with Silicon Valley’s chest. We look forward to watching her conversations with Walt Mossberg’s sweater.”

First off, I am 5’2″ (really!!) and not a “little person” as one VC once called me.

Second, Walt never wears sweaters.

And third: Chest-level was the plan, if you must know!

Silicon Valley’s chest=big traffic!

Or as Valleywag overlord Owen Thomas added to the post in note wherein he also harasses my innocent employee, Digital Daily’s John Paczkowski:

Rowr.