I was in two different Best Buys today, and there were various Guitar Hero/Rock Band kits literally all over the store. Including in places that had nothing to do with console sales (like music stuff, DVDs, etc.) just crammed any place the store had extra retail room. And there were some massive pyramids as well in the middle of empty floor space - right next to the mountain of PS3s and 360s.
I think retailers expected Guitar Hero to be a golden calf that would never stop selling... unquestionable that a lot of stores are ridiculously overstocked. Poor Gamestop had so many kits crammed up against the front window, you could barely see into the store. World Tour hasn't sold nearly as well as GT III did either, although that's what Activision gets for jacking the price up so high and releasing 5 different Guitar Hero games in barely two years.
When the expenses come home to roost, someone's going to be in serious pain.
EDIT: Oh, and a product always sells "no matter what" until it doesn't. Try telling that to Activision's great cash cow of a decade ago, the Tony Hawk series.
End of 2008 totals: Wii 42m, 360 24m, PS3 18.5m (made Jan. 4, 2008)