I know you can cancel if the powerbuy isn't complete, because I've done it. Just say you pressed the button by accident. They'll give you some spiel about how bids are binding, but they'll cancel this one time for you, but it works.
I don't know about after the powerbuy. Maybe... just call them.
I was home from work today to sign for my replayTV (and Panny DVD player!). Both are AWESOME! ReplayTV was out of the box and completely set up in 20 minutes. They give you an amazing assortment of cables (except for coax).
I just paused live tv!!!
l2c