Funny. EA's just released a $50 standalone microphone for Rock Band in the US, and may have unwittingly given the game away as to a Wii version. As seen in this report, Wii is listed as a format on the peripheral's packaging. No Wii version has been announced to date. EA hasn't commented on this yet.
Rock Band's proving bonkers popular in America, with more than 2.5 million songs downloaded for the game within eight weeks of release last Christmas. A European release date is currently listed as "Q1".