Ars Technica's Ben Kuchera writes in Opposable Thumbs:
"The only problem (with Rock Band) is that, while the Xbox 360 version of the game allows you to use Guitar Hero 3 guitars, there will be no standalone guitars sold until next year. How the hell are we supposed to play with a full band then? Ryan Shrout, owner of PC Perspective and a sneaky guy, give me a buzz to explain a way to cheat the system. I'll let him explain it, and let you argue the ethics in comments."
Here's the trick to get a second guitar for your Rock Band...
(Note: You will still have to pay for it - but that is the only way to "buy" an additional guitar, for now.)