The way they'll have to deal with this is not roll back the buy-side of a transaction if it was withdrawn. Roll back the sell-side and cover the difference. I.e., if market price is 17 and you bought at 12, MTGOX will have to refund the BTC to the rolled-back seller from the MtGox stash or, if stash is too small, add $5 per BTC to your $12 per and buy them back on the open market then refund to seller.
No doubt some buyers withdrew, but if it isn't a huge percentage then MtGox should be fine to cover the loss from the fees its collected so far. But we'll see what actually happens...