Buying music sans a physical backup (as in buying from iTunes) not your bag? Now you'll have an alternative to audio CDs. And no, I don't mean vinyl. SanDisk today is launching a brave new experiment in music sales: slotMusic, which puts audio tracks on a tiny microSD card and which will be sold, preloaded, at retail.

This isn't some podunk trial project: All four major music labels are onboard for the launch, as are Best Buy and Wal-Mart.

Why microSD? The format is being plugged as an alternative for users with music-capable mobile phones, many of which have microSD slots that go largely unused. A consumer can buy a card, pop it into his phone, and be immediately up and running, so there's no need to wait until you're back at your computer to purchase tracks online, and no need to rip a CD. (On the other hand, you will still have to actually physically purchase the music, but if you're out and about already...)

slotMusic will include DRM-free MP3 tracks, typically encoded at 320kbps, so users will get high fidelity music without having to worry about downgraded audio. Most slotMusic cards will be 1GB in capacity. Buyers will be free to back up the music to their hard drives at any time (each card will include a tiny USB adapter; an example—not slotMusic, but close enough—is pictured above), and of course they can overwrite a card later with other data when they're tired of listening to the music on it. Pricing will be dependent on retailers, but SanDisk tells me it will be competitive with CDs and "as expected."

Initially you can expect regular albums to be ported to the slotMusic format, but depending on its success, we might see labels making custom "mix cards" of popular tunes, which would seem to fit well with the grab-and-go consumer who's looking for something to listen to on his phone.

The million-dollar question, of course, is whether the market is big enough and interested enough for slotMusic. I'm a big fan of alternatives in the audio market, and slotMusic so far is hitting all the right notes—no DRM, high bitrate, ostensibly reasonable prices—but I wonder if consumers haven't moved past the "one piece of media, one album" mentality altogether. Also, swapping microSD cards on many phones can be a real pain (some have the card located underneath the battery, ugh), and the tiny cards will be incredibly easy to lose. No one will travel with more than one card in tow for fear of losing it along with his spare change. And even the cheapest MP3 player can crank out hundreds of songs instead of the mere dozen you'll get on a slotMusic card. If you meet a specific set of criteria—need music right away, have no MP3 player, and have a microSD slot on your phone—slotMusic will be a good solution for you. How many people that really comprises, I don't know.