M4P files are nasty little beggars. Only iPods can play songs with iTunes' DRM protection, so forget it if you want to port your iTunes songs to a PSP or a regular MP3 player.
iTunes already stocks certain albums free of DRM protection; they may be simple MP3s, but it's more likely that they're M4A files. Whether or not you can change an M4A file into an MP3 by changing the file extension I'm not sure.
It's generally inadvisable to include licensed music in your games (which I assume is the reason you're asking.) If you leaf through Venetia's copyright law thread (and a couple other threads that have popped up around these parts,) it should be clear enough that licensed music is going to give you nothing but trouble. Seriously, it's bad news.