I think I get what's happening: MtGox opening the accounts and blocking trading is causing many users to withdraw their BTC and sell them on TradeHill

the latter has dropped from 17 to 15 in about two hours

by the time MtGox re-opens trading there won't be much BTC to sell so at least prices on the Gox should stay relatively stable

of course, this is just a quick theory based on observation