wow, If mtgox do it this way, which is really efficient,  I will just check with my friend. I guess it's the very possible explanation.

thanks!

P.S

 If so, the Mtgox act as a trusted third party, maybe they should give the payer something can be used as evidence as payment-made, like a (mtgox address) in the account history. I guess it's only several lines of coding, right?