Ryan Shrout owns PC tech site PC Perspective. He is also a scoundrel. Guy's found a way to cheat Harmonix into sending you a second official Rock Band guitar, so that loser in the corner at your Rock Band parties, who you'd only trust to play bass, can be included in the festivities. Here, straight from Ryan's hand, is how you do it:
Go to the Rock Band support page and tell them you have a broken guitar. Then, use the shipping method that requires a credit card to ship you the guitar first before returning the bad one. It tells you the card is used to "charge you if you don't return the broken guitar within 20 days" I think. So, when you get the second one, don't return it. Blam-o, two guitars and its not stealing, just cheating the system. Of course, no idea how much they charge yet...
First in, best dressed. Mostly because there's probably not many of the guitars in stock, but also because you can bet they're sewing that loophole shut as you read this.
Need a second Rock Band guitar? Here is one way to get it [Ars Technica]