I've been told that you can only use music you buy off itunes but having never owned one myself I couldnt tell you for sure.
Just get an mp3 player that is guaranteed to work on everything. Such as just about anything other ipod but mainly iriver. Those things kick butt. Just plug them in and they show up like a regular pen drive. Copy your mp3 collection over directories and all and off you go. No crapy app between your music and player.
<A HREF="http://www.folken.net/myrig.htm" target="_new">My precious...</A>