If you trade your Mtgox BTC to Bitcoinbuilder and Mtgox declares bankruptcy, what happens? Briefly looked over the website and found nothing.
I would assume they would return them to the actual owners. Not that it would matter much...
Mtgox BTC would be $0, so Josh and all the people who fulfilled buy orders would be screwed. But would Josh fulfill his transactions to those who had successful Sell Orders on his site? That's the real question.