Analysis. Some account holders at BTC had lots of bitcoins and wanted out. They withdrew, transferred to Tradehill and sold. (Why would they delay, if they were panicky?) Thus, the price has dropped a bit from $17 at Tradehill down to $15. Thus, the sellers of BTC have already cashed them out.
That leaves people with lots of cash in Mt. Gox. Some want out and some are content to leave it parked there or to play the market there. Those who leave their cash in Gox will, if anything, push prices up when trading resumes. Others want to get their cash out ASAP. They will either use Dwolla or wire service from MtGox itself, or if they don't trust Mt. Gox they will buy BTC, transfer them to Tradehill or some other exchange, and sell BTC there and get wire / Dwolla from the other exchange. Either way, it's no difference to BTC price, as they're moving dollars to dollars.
tl;dr -- the selloff has already happened.
I don't know. A prediction this detailed requires supporting data. Right now this is just a load of conjecture.