According to Kotaku:
A friendly retail tipster sends us word that Rock Band 2 has suddenly popped up in their computer system. Better still the listings for the Playstation 3, Wii and Xbox 360 versions are all dated:
Xbox 360- September 14th
PS3- October 19th
Wii- November 18th
Why each release date is split a month a part? No idea.
http://kotaku.com/5032686/rock-band-2-dates-slip-out