360>PS2>Wii>PS3 

It is hard to tell. GHII was released for PS2 last year and it has a much larger base whereas GHII was released on 360 in March. However, the PS2 did get an update this year. Also, doesn't Rock Band come out a couple weeks early on 360 when compared to PS2 so maybe some PS2 owners will be more likely to get it. It really comes down to the large base of the PS2 vs. the higher income base of the 360. Personally, with the PS3 moving away from BC, I think it is poor planing for the future to buy anything but budget games for the PS2. People that buy GHII or RB on the PS2 may need to upgrade to GH5 or RB3 on a better system in a couple years, anyway.

I already preordered GHIII on Wii for under $75 and am very excited.  I'll buy GH4 next year and it will come with my second guitar.

I'm gonna say 360>PS2>Wii>PS3

I think Wii and PS3 will be close though. A certain amount of PS2 users will probably go to Wii as well as PS3, for different reasons.

PS3 will have downloadable songs...but the Wii will have rumble, built-in sounds from the Wiimote...I have a feeling that might be of interest to some new buyers. Both have online play at least.

But 360 and PS2 have an installed base already with the GH2 players...they won't need to rebuy guitars, and that's a big factor.

I really want GH3 for Wii...but it's either paying quite a bit for the game and an extra guitar, or just playing it once in a while on my friend's PS2, who isn't really around much anymore.

360>PS2>Wii>PS3

360>Wii>PS2 (all 3 very, very close)>>>>>>>>>PS3

360
ps2
wii
ps3

First week: 360>PS2>Wii>PS3

First Month: 360>Wii>PS2>PS3

As for numbers: A LOT

ps2>360>ps3>wii

Seeing as how the games so far have sold the best in the US, I expect the 360 or PS2 to do the best. The Wii has a good chance of beating the PS2, and a very slim chance at beating the 360, but the PS3 is pretty much guaranteed to sell the least.

This is the first Guitar Hero game on a Nintendo console, so it's still a mystery how well it'll do.

(edit: wtf, there are two separate PS2 games called Guitar Hero 2 and Guitar Hero II on the million sellers list, both with different numbers and publishers, yet the same box art)

360 > PS2 > Wii > PS3.

I think the Wii has a shot here to surprise everyone, though. There's been a lot of talk about how Wii owners do not buy third-party software, but in their defence, a lot of their third-party offerings have been godawful. I think this game probably would be a fantastic fit for the Wii's audience and it might end up leapfrogging PS2. Then again, it might inexplicably be completely avoided. Besides Mario and Zelda games, I really find it difficult to gauge Wii owners' software purchase habits.

Well...remember the predictions made for Madden...Wii sold the least by FAR.

But this IS different, seeing as new guitars will need to be purchased for both the PS3 and Wii...plus graphics aren't really a big deal for people when it comes to Guitar Hero.

Downloadable content is an issue though...and it will favor the PS3 and 360 over the Wii...