Judged by all the one to multiple address transaction it should be a service provider on the Bitcoin network of some sorts, it could be mybitcoin.com , or deepbit mining pool , or mtgox , or just a rich paranoid privacy concerned individual splitting and consolodating and transferring his BTC. Very unlikely, but investigation is warranted, someone could have double spend himself up to that amount by gaining control of the network with a botnet - but then that would have shown as a spike in the network hash here when it happened:
http://bitcoin.sipa.be/Also if you add all the major mining pool's network hash power (see bitcoinwatch.com) - it equates roughly to the network total, and nobody at present has a majority to override network algorithm rules and accept his own double spend. This again shows how important it is for the network integrity that it is decentralized and not monopolized/centralized under one major mining pool.
Or it could be something/someone hiding and double spending in the "Other" category of miners as shown on bicoinwatch.com , doubt that though - it is only fairly recent development when deepbit mining pool went down, but definitely worth inspecting by the blockchain inspectors!!
