I've seen work of several online payment systems and had money on accounts of two of them when they gone. Before they gone: they made holds on higher volume outgoing payments(it's obvious, that they didn't have enough money to make transfers instantly, so they queued payouts). And that's what Mt.Gox is doing right now with wire transfer payouts(most of higher volume payments are paid using wire transfers). And I've heard from people familiar with owners of one of those payment systems, that they "invested" clients money(it's not really hard to beleive in, since almost everyone does that. Not everyone is skilled enough to measure risks correclty). Or they can just do something stupid
, like Pavel Vrublevsky did with Fethard. And it's very likely, that Mt.Gox doing something similar, since higher volume payouts are not processed instantly. Be careful with Mt.Gox and everyone else, who delay payments.