Rock Band is coming out on the Wii June 22, with the guitar-mic-drum-game bundle price of $179. You've played it on other systems. I played it on the Wii. My short impression: It's Rock Band on the Wii.
Rock Band looks a little different on the Nintendo system. The controllers are all finished in white and mimic Nintendo's blue LED dots to mark the player order. Best of all, the guitar controllers are wireless ($60 alone). [Update: A couple comments have asked good questions about how the wireless gear connects. There's a USB hub and dongle with an RF transmitter. So the Wii Remotes aren't a factor except for the mic user.] And the white drums ($80 alone) and other hardware include behind-the-scenes revisions that weren't included in the original launch last fall. Harmonix mentioned that those changes, including a wider drum layout and sturdier parts, have been rolled out to the 360 and PS3 already.
Rock Band Wii includes 63 songs, including a few "bonus" tracks that PS3 and 360 owners might have already downloaded, like "Roxanne". As already lamented, the Wii version doesn't offer online downloads or internet play, but game representatives indicated that those are important additions that they're pursuing. (Roughly $12-million worth of "important" and counting.) However, there are no specific plans to either add them to the Wii game or save them for a Rock Band 2.