Almost feels like it's one of the MtGox accounts itself. Transaction-wise, it has been making easily 200-300 BTC a day. Perhaps also doing some personal buying and selling...
Edit (added link) + post 122 as well. http://forum.bitcoin.org/index.php?topic=20207.120
It's not what Mt Gox allows that matters - it's been hacked and compromised specifically to allow people to do things that are not allowed