I mentioned it in Destructoid's Rock Band review, but I'll repeat it (and expand on it) here:
I went to Virgin Megastore in Times Square because I was looking for UT3 Collector's edtion for the PC. I didn't have any intention of buying Rock Band HOWEVER,
They have a crapload of Xbox 360 Rock Band kits, as well as the standalone game of course.
At least 11 at last count. I guess they have PS3 ones, but I didn't notice, so sorry about that. So if you didn't preorder like me, but still want it,hit up the Megastore before they run out. Which I guess will be before this Friday.
FULL DISCLOSURE: I do not work for Virgin in any way shape or form, nor do I gain anything from anyone buying Rock Band there. I'm just sayin'