The Wii, PS3, and 360 will have a COMBINED leaderboard!
LET THERE BE ROCK!!!
http://au.wii.ign.com/articles/825/825222p1.html
"So how does the web community tie into the game? Once you sign online with Guitar Hero III, you'll be able to get a unique code that will link your game with the webpage. From there, all of your stats will be available on the webpage to browse and compare with everyone. And since the Xbox 360, Wii and PS3 versions will all make use of the online community, these will be the ultimate leaderboards.
Of course there's more on the web than just leaderboards. You'll also be able to personalize your profile, create an online band similar to a clan, collect groupies for meeting specific goals, challenge other bands, and compete in online tournaments. You may not be able to directly play against the other platforms online, but Guitar Hero III will ultimately link the three console iterations together into one massive meta-game through the online web portal. How cool is that?"