Just added to Mt. Gox's API documentation is the ability to Redeem a wallet.dat, which allows a simple way to import (via upload of wallet.dat) all your private keys to be managed by Mt. Gox.
All bitcoin amounts in your wallet will be transferred to your Mt. Gox account and any amounts received later will be swept at that time as well.
Does this feature work with an encryption-protected wallet.dat though? The API doesn't show any way to send the passphrase as far as I can tell.