It lost 750,000 bitcoins to a "hacking attack", reports The New York Times, a haul worth some $400m at the present exchange rate. This site has some useful information on the cryptocurrency's fluctuating value.
Update: More from Cyrus Farivar at Ars:
Tsutomu Okubo, now a member of the Japanese parliament and a former banker at Morgan Stanley, told Ars that he had begun discussing new bitcoin-related regulation with the Financial Services Agency, the Ministry of Finance, and the Bank of Japan a year ago. "However, their response is that Bitcoin is neither currency nor regulated settlement in Japan," he said by e-mail. "Regarding exchanges like MtGox there is no regulation now."