But just barely: Digital sales accounted for 50.3 percent of all U.S. music purchases last year, according to Nielsen SoundScan.

And if you want to caveat that number, that’s easy to do: For starters, the Nielsen number refers to unit sales, not revenue. So the music company’s books might still show that analog sales make more money for them. And recall stunts like Amazon’s Lady Gaga almost-giveaway last spring, when it sold nearly half a million albums at 99 cents a pop.

Still, the big picture finally resembles the one we’ve been hearing about since Napster showed up in 1999, or at least since Apple started selling music via iTunes in 2003: One day, files would beat discs.

The milestone happens to have occurred in the same year that music sales finally ticked up again. You can pick a couple different metrics to illustrate the rise, but they’re all single-digit increases. The important point is they’re not decreases.

That could simply be a one-off, and perhaps the result of lots of people buying Adele songs — Sony’s new star sold more albums than anyone has done since 2004 — or there could be a larger change afoot. Won’t know about that for a while.

But an industry that hasn’t had good news since the Backstreet Boys were big can afford to be patient.

But! Here’s a small data point you can add to the “maybe things are getting better, or at least less worse” argument: Nielsen Soundscan says U.S. music sales (by unit) are up 1.6 percent for the year to date. It credits the increase to a boom in digital, which had been flattening out a year ago.

Again, it’s early in the year. And even if sales do end up positive for all of 2011, that’s one year versus more than 10 years of decline. So I’d be hesitant to make too much of any of this.

Still: The numbers do sync up with an earlier report I pointed out last month, which showed sales were down a mere 1.3 percent at the time. They also align with Warner Music Group’s Q1 numbers, which showed growth, too.

I haven’t heard anyone in the music industry explain why they think digital is up dramatically so far this year. Nielsen’s chart below flags the arrival of the Beatles on Apple’s iTunes last fall, but I have a hard time believing the “Love Me Do” bump extended into March and April. Anyone want to hazard a guess?

Here’s the full data dump, for those who care. And please, please, please don’t get excited about the vinyl sales increase–even after a 14.2 percent jump this year, vinyl represents less than 2 percent of sales.

Total Albums w/TEA +1.6%

(Track Equivalent Albums)

Overall Albums -1.5%

Physical Albums -8.3%

Digital Albums +16.8%

Digital Tracks +9.6%

Physical Formats:

CDs -8.8%

Vinyl +37.0%

By Current/Catalog:

Current -7.0%

Catalog +5.4%

Nielsen SoundScan says the drop was either one percent or .09 percent, depending on how you count, so this isn’t the bottom falling out. But it does look like a peak, and it has been in the works for some time.

Earlier this year, Warner Music Group (WMG) pointed out that it was seeing its digital sales slow and argued that one reason was because the industry had raised prices on most of its songs at Apple’s (AAPL) iTunes store in 2009.

That thinking is now pervasive across the industry, Billboard notes: “While consumers will still buy hit songs for $1.29, it seems that catalog tracks priced at that level are not selling as well as they were at 99 cents.”

But you could also make the case that digital tracks were going to decline anyway and that the industry is better off squeezing every penny it can.

And if you want to try to find a silver lining here, you could argue that since song sales are slipping, there’s no reason for the industry not to support rental/subscription models like Spotify, Rhapsody and MOG by cutting their licensing fees. But I wouldn’t bet on that happening soon.

But here’s a very slight twist: The sales decline slowed in the most recent quarter. U.S. sales dropped 11.1 percent in Q3, compared to a 14.5 percent drop in Q2, according to Nielsen Soundscan.

That’s the good news. The bad news is that Michael Jackson isn’t going to pass away again–and that unless they finally do come to Apple’s (AAPL) iTunes, there probably isn’t another way to repackage the Beatles again. Billboard:

Renewed interest in the Beatles and Michael Jackson slowed the decline of U.S. album sales in the third quarter, although the industry is still on track to fall for the eighth time in nine years….

Music retailers are hoping that the continued performance of Jackson and Beatles albums and a strong fourth-quarter release schedule will continue to make up lost ground.

During the quarter, Jackson’s June 25 death fueled sales of about 5 million units, and the September 9 re-release of the Beatles catalog has sold 1.3 million units so far.

So far this year 11 albums have topped the 1 million-unit mark, the same number as in 2008. In 2008, the top seller was Lil Wayne’s “Tha Carter III,” at 2.5 million units; this year’s top seller is Jackson’s “Number Ones,” at 1.8 million units.