360>Wii>PS2>PS3

Since this is the first Gh coming to Nintendo I think it'll do great.

360 > ps2 ~ wii > ps3

ps2 owners with a guitar can choose to just purchase the game itself, right?

 Currently, no.  I doubt the PS2 version does either.  The Wii does support online play though.  Actually, I could have sworn they said PS2 would also support online play...that would be interesting, though certainly less used I'm sure.

Here's my [odd] guess:

First week: 360 > PS2 > Wii > PS3
Life: PS2 > 360 > Wii > PS3

so there's over 5 million wii owners in NA and only the two usually wii games are in the top 10. You'd think these wii owners(including me) are just itching to buy a good game....

PS2>360>Wii>PS3

My reasoning is, as BenKenobi88 mentioned, the PS2 and 360 owners don't have to rebuy guitars. I believe that already having that considerable investment of a guitar on one console will essentially lock down which console you buy the next one for, barring upgrading to one of the next-gen systems from the PS2. Also, I think the built in community will influence which one is bought, so if, say, your friend has one for 360, you would be more likely to get it for that system. I think the PS2 will see a slight but noticeble decline in the end, with 360 version selling significantly more than it's predecessor, and the Wii coming in a strong third and PS3 last but still strong. For the last two, I'm working on the assumption that this is right up the Wii's alley, and for the PS3 having both a higher price of entry and lower installed base.

I really think this game is tailored for the Wii. Just think about it; the same craze that lead casuals into the Wii in the first place (Wii sports) could continue in the form of Guitar Hero. I think this game will sell tons on the Wii.