Apple put out a press release today announcing a milestone: The company has now sold more than 5 billion songs through iTunes.

To put that into perspective, if you pile 5 billion digital downloads atop each other, they'll form a tower tall enough to ... well, OK, it'll be a completely invisible tower, as digital downloads have no three-dimensional physical presence, but you get the idea. It's two or three road-trips' worth of music, at least.

But let's get back to this business of digital downloads lacking any physical presence. This idea -- the notion that music, now, is just math, just information floating about the ether -- turns out to be of some importance. Apple has sold us 5 billion songs, but do we really own that music?

Not most of it. People seem dimly aware of this, but it bears repeating: The vast majority of the songs we've bought through iTunes are gummed up with FairPlay, a digital-rights management scheme that Apple cooked up years ago to satisfy the recording industry.

FairPlay works like this: Every time you move your music to a new computer, iTunes calls up Apple's servers to request "authorization" to play the tracks. The trick works fine, usually, as long as you are abiding by Apple's restrictions.

But what if Apple's servers go down? Indeed, what if, at some point in the future, there is no Apple, or iTunes? Then you're stuck. That's the gamble of copy protection: Because your songs must phone home, they're not your songs, not really.

Why am I mentioning this now, raining on Apple's 5 billion parade? Because the concern is not hypothetical. Exhibit A: Customers of Microsoft's music store, which went online in 2004, and unceremoniously came down in 2006, are smarting over just this sort of thing.

In April, the company sent former customers an ominous notice. Microsoft had decided to shut down its authorization servers, meaning that people's songs would break after Aug. 31. The company recommended that they laboriously burn each of their tracks to audio CDs (a process that results in lower-quality digital tracks).

After an outcry, Microsoft announced yesterday that it has reconsidered its decision.

Now customers will have until 2011 to enjoy the music they purchased. But MSN customers are living on borrowed time. One day, Microsoft will power down its servers, and when songs call out for permission, they'll hear no response, and they won't play.

But this is true of all DRM-protected music, not just Microsoft's. ITunes and Apple don't look vulnerable now, but the tech industry changes fast. One day a company or a product seems invincible, the next it's curtains.

There's no reason to gamble: When you're looking for digital downloads, check out Amazon's superb MP3 store first. And if you must buy from Apple, make sure your track is labeled "iTunes Plus," which is Apple's way of saying it's free of copy protection. Your music shouldn't have to ask for permission.