The cool thing abou the Wii is the Wii-mote makes for these type of "peripheral" games to be Very cheep compared ot the others. All they really are doing like with GH3 is extending the Wii-motes buttons, your not buying a whole extra controller like with PS/Xbox, its just another attachment to the wii-mote.
Normally a differance of atleast 10-20 dollars in price although with rockband already being out I would bet it hits the wii at a nice price range of maybe 139.99 or lower. Although you will need atleast 2 wii-motes for it.
So far rumors have been summer-fall of 2008 but thats rumors. I'm sure they will rush it out as soon as possibal, probably be a Pay-to-play game because of the DLC on it, but you pay for the DLC on all versions of it. Beyond that it should be identical to the current version just toned down graphics wise.
I will get it for sure, would be nice if they have the guitar compatability issue between GH/Rockband sorted out before this hits the Wii. Just use the GH guitar setup for Rockband aswell, that would be perfect! Give me an extra guitar for GH and all 4 instruments for RockBand.