Activision made a statement over the weekend about the nature of their Guitar Hero: World Tour "track pack" downloads and what it means for the Wii version. Pressure from two outside parties has constrained the DLC catalog, unfortunately. Some labels or artists will only give Activision the rights to distribute track packs, with individual sale of the contained songs prohibited. And for some reason, according to the statement, "the Wii's infrastructure only allows us the ability to release single songs as DLC."
That means that some artists will have to be excluded from the Wii's DLC offerings entirely. Like, for example, Jimi Hendrix. Because of these restrictions, none of the Jimi Hendrix tracks from the PS3/Xbox 360 set will be available. So it won't just be the limits of your ability that keep you from playing Hendrix songs.