I'll be waiting for this before I get a Fitbit. And excuse the imagery, but it seems you have to wear clothes in bed for the vibrate-to-wake feature to work with the existing versions ;)
At night the tracker is within the sleep wrist band, so it vibrates near your wrist. So feel free to pull a Marilyn Monroe. :-P
Actually, it comes with a wristband to clip it to for sleeping.