It looks like the hacker who breached Mt. Gox made off with about $34,000 worth of Bitcoin and then artificially crashed the market by dropping a sell order for 500,000 BTC, according to the post-mortem about the hack published by Mt. Gox. But while the hacker did withdraw 2,000 in actual BTC, which Mt. Gox is replacing at their own expense, the enormous sell order was vapor:
We would like to note that the Bitcoins sold were not taken from other users’ accounts—they were simply numbers with no wallet backing. For a brief period, the number of Bitcoins in the Mt. Gox exchange vastly outnumbered the Bitcoins in our wallet. Normally, this should be impossible.
The sales could not have been completed because there were no actual Bitcoins to transfer. The hacker had simply assigned himself a huge number of BTC, which was enough to place orders on Mt. Gox and confuse the market. Read More