Problem is, (for PS3, I hear this is NOT the case on Xbox) that the guitar from GH will not work on RB, and they do not sell just the guitar. Boo!
Means you have to either, invite friends with a guitar who also have RB, OR, buy another whole set just for the guitar. Kinda sucks doesn't it.
Hopefully the publisher who's name escapes me at the moment (I think Activision) will get their head out of their ass sooner rather than later.

Rock Band is developed by a company called Harmonix, which developed Guitar Hero 1 & 2. Activision bought the company Red Octane which developed the Hardware for Guitar Hero, and MTV bought Harmonix. So they no longer work together and Harmonix did not develop Guitar Hero 3.

At this point, both sides are kinda pointing fingers at each other as to whose fault it is that the Les Paul PS3 controller doesn't work, but Harmonix has developed a patch that would fix it, but Activision is blocking the release of the patch. ( wiki info )

Full Disclosure: I'm a little quick to defend Harmonix as my college roommate is a developer there.

I'd defend Activision but I used to work there :D

wow. it'd almost be cheaper to take up really playing the instrument, wouldnt it

(dont mind me. guitars I have, but at guitar hero, I stink on ice)

Both of which where blatantly stolen from Konami's Guitar Freak. I have been familiar with Red Octane long before the big GH craze. They were an online retailer who sold Dance Dance Revolution games and 3rd party dance mats. They also published In the Groove, a ripoff of DDR. Red Octane are pretty much thieves.
But anyway...;)
PS, I'd defend Harmonix too, Activision should make a guitar you can buy separately or stfu. just sayin'

I would defend either Activision or Harmonix if I cared.

So why did you post?

I want it- gonna have to look hard though. You can find it for PS3 just about anywhere- but not XBox and that is what we want it for.

hmmmm..bonus coming up, tax refund, trip to Disneyland and maybe some new high tech toys?

Yup. And I am not buying EITHER till this issue is resolved.

F**k both of em. Seriously.

Honestly, I'm not really sure what Harmonix has done wrong. They developed to an open standard. Every other guitar on the market works. Sounds like Activision's problem to me.