Rainbird on 28 February 2009
Both Eurogamer and Joystiq says april 24 for the Wii version.
They also mention new prices on the instruments
"The prices of instruments will also be revised: standalone drums will cost GBP 49.99, guitars GBP 39.99 and a full "instrument edition" set will set you back GBP 89.99.
There'll be official software and hardware bundles for the first time in the region, too. These come in at GBP 119.99 for PS3 and Xbox 360 and GBP 109.99 for PS2 and Wii. "
http://www.eurogamer.net/articles/rock-band-2-dated-for-ps-and-wii
