Chris Foresman on iTunes Match:
>iTunes Match will let you mirror up to 25,000 tracks in your iCloud, and those songs can be pulled down to any iPhone, iPod touch, and iPad, as well as synced with Macs or PCs running iTunes. This includes tracks ripped from CDs or downloaded from the Internet, even those you may have obtained in a less-than-legal manner.
Well there goes my theory that it would only work with tracks ripped by iTunes — this is a great read if you are interested in the one thing we can’t beta test right now. Sounds like a killer service.