Look for the iTunes Match icon on the left hand side of iTunes, assuming it's up to date with at least 10.5.2, and then opt in for $27.99 per year to have all the music in your library in the iCloud - whether you purchased it through the iTunes store or not.
The other perk is that you get all your music upgraded to 256Kbps DRM-free, even if you downloaded it from some crappy Russian MP3 site years ago. cough allofmp3 cough
That's a sarcastic finally in case you can't tell. Sarcasm is every Canadian's middle name, but it's hard to tell online. ↩