"Bottle, Barbies, & Boys" has a great post up right now where Johnina discusses her experiences as a sleepwalker and the mother of sleepwalking kids. It is both scary and entertaining.
My youngest sister used to actually climb the stairs in her sleep, too. For some reason she only tended to go into two rooms, mine or my parents. We could always tell when she'd gone into my parent's room because she'd stand over them and talk and my mom would wake up screaming at the top of her lungs. Eventually, my parents put a front door type of lock on their bedroom door just to make sure that they could, uhh..."cuddle", in peace without a sleeping kid creeping in and babbling incoherently.