I saw two different spins on this issue yesterday.
EB Games/Gamestop says that you can not play any of the GH games on the PS3.
Red Octane (as of early September) said you can play GH 1 with the new PS3 guitar and there won't be any problems. You can play GH2/GH80's, but the hammer ons/pull offs, tilt, and whammy will be off, and probably won't work correctly.
I was really hoping that GH3 would solve the problems, but it doesn't look like it's going to do that. I planned on getting GH3 just for the additional guitar for Rock Band so I'd have something for a bass. Harmonix hasn't said anything about their guitar for Rock Band, but possibly that might be able to give you full use of all GH titles.
Since you can't properly play GH1&GH2 on PS3, I'd assume that Red Octane plans to package the old songs as downloads, like what they did for GH2 on 360.