Send the bitcoins back to MtGox to repeat the trade and you'll have your answer.
Right. Notice that by the time the value transferred, it was too late for arbitrage.
There are at least two people running arbitrage 'bots. They maintain balances in both USD and BTC on both Tradehill and MtGox. So when a price difference bigger than the commissions involved appears, the 'bots pounce on it immediately. Unless the arbitrageurs run out of USD or BTC on one exchange or the other, the exchanges don't stay out of sync for long. Arbitrage opportunities last about an hour.
(Could be worse. Arbitrage opportunities between exchange-traded funds and the underlying stocks last milliseconds