Take heart, PS3 Rock Band owners: To hear one Harmonix poster tell it on the game's official forums, you're just moments away from being able to mount your own four-piece outfit. Harmonix staffer "Sean" writes that a patch should arrive today that will enable Guitar Hero III controllers to be used with the game, which, as Rock Band guitars aren't available individually, will be the first time that four-player action will be feasible.
The only question mark now is exactly when the update will become available. Sean writes, "We're trying to find out what the holdup is and I'll get back to you when I know." For now, it looks like you should just keep your rocking hand well-oiled and keep an eye on this thread.