If you submit a withdraw from MtGox and let's say the price is at $500 and they don't "process" your withdraw until 2-4 weeks later, at what price are you being locked into? Are you being locked into the price on the day you initiated the withdraw, or on the day that the actual withdraw takes places.
This is a no-brainer. You are locked to the price which you chose to sell your BTC for. There is no "withdraw BTC as fiat" option. MtGox will not reduce your amount if fiat if the price is lower than it was when you chose to sell.