Oh, goodie. I've yet to take my Rock Band gear out of the box, but plenty of you have and are now experiencing hardware problems with the included Fender Stratocaster controller. The majority of the problems reported on the official Rock Band community forums indicate that the controller's "down strum" simply won't register or will double register notes after a few hours of playing. Some are reporting bad hardware right out of the box, but general user feedback indicates a progressive issue.
Official response on the forums has been that users experiencing hardware problems should recheck their connections and restart the game. If Rock Band hardware is still failing, users are being directed to EA's warranty page, to request a replacement controller. Good luck and godspeed, replacement guitars.
The OFFICIAL "One of my Instruments is Already Broken" Thread [Rock Band Forums - thanks, All!] Rock Band Warranty [EA]