The asset freeze at Mt. Gox was due to the Bitcoin exchange's failure to obey financial regulations as required by US authorities. The news comes via IDG, which obtained a copy of the seizure order from the US Immigration and Customs Enforcement (ICE).
The agency froze the Dwolla (a US-based online payments system) account of Mutum Sigillium (aka Mt. Gox) on the grounds that it had lied in an official form. When asked if his company "[accepts] funds from customers and send[s] the funds based on customers' instructions," Mt. Gox CEO Mark Karpeles answered "no." When asked if Mt. Gox "deal[s] in or exchange[s] currency" for its customers," Karpeles again answered "no." In both cases, it seems likely — and ICE asserts — that these answers were incorrect. Mt. Gox is the world's largest Bitcoin exchange, and facilitates a large number of transfers of the virtual currency.
The two answers could land Karpeles and his company in a lot of trouble — the failure to register as a money transmitting business in accordance with US law is a serious crime. Violators are liable to be fined, and could face up to five years in prison. We've reached out to Mt. Gox for comment on the matter, and will update you when we hear back.