Unless you check the Open Transaction box on the withdrawal screen, if the address you enter is connected to another MtGox account, it will be deposited into that account instantly and without hitting the blockchain. If you have a transaction that needs to be public, always check that box just in case the receiver is using a MtGox address.
Instawallet does the same thing, minus giving you the option to put it on the chain. Actually, now I'm not sure, maybe it's just that it puts them on the chain and accepts them with 0 confirms.