Are these predictions worldwide or for the Americas only? I know the games don't usually sell much in PAL first week, but it's gaining steam there. I'll just go with America.
Wii: 750k
Xbox 360: 600k
PS3: 400k
PS2: 320k
That's assuming that the franchise actually grows from last year. The higher price tag for the extra instruments may hinder sales a lot. I'm probably being fairly optimistic for the sales, but I've already typed up this entire post, so I may as well enter it.