With all this recent talk about Bitcoinica, I couldn't help but do some research on prior Bitcoin large thefts (as of now, defined as over one thousand bitcoin), all during the period I was myself involved with Bitcoin but did not know about (March 2011 was the month I first learned about Bitcoin). Before that time, I assume the thefts were not well-publicized or were minor in nature. I have provided dates and UTC times as I know them, and to be as accurate as possible. For disputed thefts, I applied best judgement and included the ones that were most publicly accepted.

Because of the volitile nature of Bitcoin's exchange price, I have denominated heist estimates in BTC. Although not heists per se, major permanent bitcoin losses are also included here in italics. If I missed any major thefts, heists, or losses, or if you have any other information to contribute to one of these events, please leave a reply in this thread.

Note: To qualify, a major heist, theft, hack, scam, or loss must cause damage greater than or equal to 1000 BTC. Borderline thefts may qualify if reasonable estimates are over or equal to 1000 BTC.

List of events by severity (BTC)
Listed by the name found later. Use Ctrl-F for more information.

1. July 2011 MyBitcoin.com Theft (78739.58205388 BTC)
2. March 2012 Linode Hacks (46653.47830495 BTC)
3. June 2011 Allinvain Theft (25000.01000000 BTC)
4. April 2012 Silk Road Scam (20000 BTC)
5. May 2012 Bitcoinica Hack (18547.66867623 BTC)
6. August 2011 Bitomat.pl Loss (17000 BTC)
7. October 2011 Bitcoin7 Hack (15000 BTC)
8. June 2011 Stefan Thomas Loss (7000 BTC)
9. February 2012 Bitcoinica Theft (5000 BTC)
10. September 2011 Moonco.in Hack (4000 BTC)
11. June 2011 Mt. Gox Hack & Theft (2643.27 BTC)
12. October 2011 Mt. Gox Loss (2609.36304319 BTC)
13. 2011 Ubitex Scam (1138.98 BTC)
14. 2012 Bitscalper Scam (1000 BTC)