Apparently all those downloadable songs you purchased for Guitar Hero 3 are going to work just fine with the upcoming Guitar Hero World Tour (aka Guitar Hero 4). But not, apparently, with the rather redundant Guitar Hero Aerosmith.
Guitar Hero World Tour will copy competitor Rock Band by including drums and vocals as playable instruments. Presumably the Guitar Hero 3 content will be guitar-only when you import it into the new game. That's still pretty awesome, though.