Well, the Best Buy in Tupelo, MS actually set their Rock Band demo up on Friday, and I just so happened to stop there on that day. It was amazing and you know all that stuff, but I did notice something interesting. Different songs from the game were playing while the demo was on standby, such as "Go with the Flow". At one point I heard "So Sick" play.
So, I'd say that pretty much confirms that rumor. Someone else go hang around the demo to back me up.
Hope this hasn't been discussed, btw.