According to a user on BitcoinTalk, around $500 million worth of Bitcoin has been stolen since August 2010. This includes Bitcoins stolen from Stefan Thomas, Bitcointalk.org user Allinvain, Mt. Gox, MyBitcoin, Mooncoin, Bitcoin7, and Silk Road. Each of the entries in the forum includes the amount of money stolen.
The entries total to around 818,485.77, which is worth over $500 million. There are around 12.4 million Bitcoins in circulation, which means that 6.6% of the supply was taken through illegal means. One out of ever 16-17 Bitcoins belong to someone that stole it. One of the largest heists involving Bitcoin was through Mt. Gox.
Around 750,000 Bitcoins were stolen from Mt. Gox customers and the company lost 100,000 of its own. The currently that was taken from Mt. Gox was worth around $473 million.