Mark Pilgrim has written a very cogent, readable essay on Microsoft’s consistently lame music offerings:
Microsoft named this developer platform “PlaysForSureâ€Â, and they (and their partners) ran many, many ads decrying the fact that music purchased from Apple’s iTunes Music Store would “only†play in iTunes and on iPods. This was, technically speaking, true  and indeed it is still true, and it is why I have cautioned Dora and you and anyone else who would listen that you should never “purchase†anything from the iTunes Music Store that you might want to “own†longer than Apple was willing to allow.
I can’t get very excited about digital rights these days, but this is a well-written explanation of why this stuff might matter to the average Normal Human.
DarrenBarefoot.com 
A minor addendum to Mark’s blanket equivocation of iTunes with Microsoft’s DRM strategies: iTunes+ tracks are the same price and come with no DRM. The availability of an iTMS+ track is dependent on the license holder.
This is hardly “hot news” any more, a year later, but here’s what Steve Jobs actually thinks of DRM:
http://www.apple.com/hotnews/thoughtsonmusic/