Mac expert John Gruber spells out the implications of music subscription services like Super-Bowl-advertised Napster To Go: it's renting, not buying music.
Napster to Go lets you listen to unlimited music for as long as you are subscribed, for 180 bucks a year. When your subscription runs out, the music is no longer accessible. The "Do the Math" campaign which compares iTunes Music Store purchases to Napster to Go really compares apples to oranges, renting to buying. (And both have strings attached. Then there are the no-strings-attached options...)
Subscription Small Print [Daring Fireball]
Related:
Forego the math