If MtGox could make it so that you could add your PGP public key and then configure your account such that bitcoin withdrawals require PGP signature of a pre-generated message that contained the destination bitcoin address, MtGox would have undisputable conclusive proof in the event of a disagreement as to whether a withdrawal was authorized.

Put another way, if MtGox's withdrawal had just the same security we have on our IRC channel we use for chatting, confidence would be increased, as we'd have less fear of being stuck in a situation where money has been withdrawn with no way to convince anyone that we didn't do it ourselves.

I say first hand that anything that can be done to increase the confidence in security of funds stored in MtGox will directly correspond to a greater willingness to leave funds in MtGox.

In fact, implementing this idea would put MtGox in an even better position: in the event a hacker really managed to compromise a PGP key and forge a signature on a withdrawal, I think most people in this community would consider it 100% reasonable for MtGox to say "here's his signed request...sorry he's SOL!...do a better job of securing your PGP key next time"... far more than "sorry you must have gotten keylogged or something."