The Guitar Hero 2 Xplorer controller works with everything. We tried it out with Rock Band, Guitar Hero 2 and Guitar Hero 3 on Xbox 360 with no issues whatsoever. The only drawback to this controller, aside from its wired status and button placement, is that you can not activate any of the extra guitar effects featured in Rock Band. For that, you'll need the Rock Band Stratocaster.
The Guitar Hero 3 Les Paul also works with everything and also can't be used to activate the Rock Band special effects. We tried it on Rock Band and played through roughly 10 songs without any hitches. Of note here is that the Guitar Hero 3 PS3 controller can't be used with Guitar Hero 2, even with the disc put into a PS3 that should make it backwards compatible.
The Rock Band guitar is another story. This controller, which features a number of extra fret board buttons and an effects switch, does not work with Guitar Hero 2 or 3. Cycling through the menus works just fine, but as soon as the game begins it all breaks down. Buttons don't register at all and the game is impossible to play. In addition to the extra buttons, it has one other advantage while playing Rock Band. During extended "jam" sessions of the game where you can press any buttons you want to boost up your score, you don't have to strum when using the Rock Band Strat. With the Guitar Hero 2 or 3 controllers, strumming is required.
You can't buy Rock Band without getting the guitar in the bundle this holiday season, though you can get Guitar Hero 3 without the wireless Les Paul. To that end, it seems that the only real decision people who want both games have to make is whether to get the Les Paul or stick with the Guitar Hero 2 Xplorer.
We haven't been able to test the PS3 Rock Band Stratocaster with Guitar Hero 2 or 3 just yet.
It should be noted that things may change before Rock Band is released as the game and, perhaps, the controller aren't finalized. We went to Harmonix and RedOctane/Activision for official word.
Activision had no comment at the time of this publishing, though promised one in the near future as it is still being put together. A Harmonix Spokesperson gave this statement to IGN:
"Harmonix develops and bases all of our games, guitars and other peripherals on open standards established by platform manufacturers such as Microsoft and Sony. Rock Band is no exception and supports an open controller standard. Guitar controllers are like any other standard game controllers--the controller sends a signal back to the game depending on the buttons you push. If a third party game controller is based on open standards, then it should work with Rock Band as well. If certain controllers do not work with Rock Band, questions about those controllers should be directed to the peripheral manufacturer. In addition, we welcome any 3rd party software that wishes to support Rock Band guitars. If a 3rd party guitar / music game does not support the Rock Band guitar controller, questions regarding this should be directed to the game manufacturer.
The official Rock Band guitars and peripherals are custom designed to help players to blur the line between playing a game and being in a band. Harmonix has modeled the guitar on a Fender Stratocaster and engineered the strum bar so it feels like you are playing a real guitar. We have ten fret buttons--5 on the top and 5 on the bottom---and have even built in a five-way effects switch that interacts with the game. Obviously the players have a choice, but we truly feel that once they play Rock Band with the Stratocaster, they will not want to use any other guitar controller."