Rock Band Stage Kit must accept that the device will not be compatible with The Beatles: Rock Band. Harmonix has confirmed to Joystiq that there are "no plans to patch it for The Beatles: Rock Band." As to whether the device will be compatible with future editions of Rock Band, a company rep stated, "We haven't announced any future titles at this time."
There's been some back and forth on the Rock Band forums about whether the smoke machine and disco ball-ish device -- originally $100 (but cheaper now) -- would be patched into The Beatles game. We hope this settles it -- not that you have a choice, apparently.
[Thanks, Jeremy F]