They put your bitcoins in as a Mt Gox trade to equal the $ owed + 1%
As soon as Mt Gox executes it, you have paid. Hopefully the exchange rate wont drop like a stone, like it did for me, but it bounced back up the next day.
So I put in an amount of bitcoins at equal to what I hope will be the rate + 1% a short time in the future and hope for luck?
Although my Mt Gox account just got verified I havent used it yet so I only partly get this.