I have a deposit address at Mt. Gox. Say I send 10 BTC to that account. The 10 BTC will be sent to *my* Mt. Gox receiving address. Now, say you send 1 BTC to SD. If it uses some of my 10 BTC, which it might, then your transaction's sending address will be my transaction's receiving address. To Mt. Gox, it will look like I am depositing more Bitcoins.
Thanks Joel. That was a very clear explanation.
It unfortunate that the Mt. Gox reps were unable to come up with any explanation.