According to MtGox, the hacker got about 2000 BTC. A few real customers also appear to have purchased bitcoins at an artificially low price during the incident, withdrawn them, and kept them. This is estimated to amount to less than 650 BTC.
The details given at the above link are:
~June 2011 Mt. Gox Incident
Time: 2011-06-19T18:00 ± 1 h (theft), days ensuing (hacks & withdrawals)
Victim: Mt. Gox (some claim also customers)
Status: Thief uncaught
Stolen by thief: 2000 BTC (official estimate)
Additional withdrawn from Mt. Gox: 643.27 BTC (lower bound)
Total: Lower bound 2643.27 BTC
Equivalent USD: 46970.91 $ (used last trade value; actual price at the time was not reliable)
Transactions: none released officially
The event's scale was widely disputed; some report a theft of almost 500000 BTC due to related account hacking. At this time, these reports seem to be unfounded and closer inspection puts the losses at closer to 2500 BTC. Some additional money was stolen by traders capitalizing on the dropping price and withdrawing in time, including toasty of the bitcointalk.org forums. Mt. Gox claims it has reimbursed customers for this theft.