Spreading your songs across devices is going to be very easy. "iTunes in the Cloud", part of the large (and free!) iCloud package, will let you re-download any Apple-purchased song to any device. But non-iTunes buyers can join in too.
If you buy a song from Apple on your iPhone, download it to your iPod. Buy a song on your iPod, download it to your iPad. Set it to push automatically, or pick whichever tracks and albums you'd prefer.
Or, if you're like a lot of people and haven't built your entire library from Apple's trove, there's iTunes Match. And this is the big one. For $US25 a year, Apple will upgrade the crossover between your library and theirs to 256 kbps AAC files, and offer the same re-downloadability as if you'd copped it from Jobs - and no storage cap. Will this work with massive amounts of pirated music? We don't see how it couldn't. So whether you've amassed a giant library from CDs or the seedier corners of the internet, Apple's giving you unlimited access for 25 bucks a year. Someone at the RIAA just punched a hole in the wall.