Mt. Gox isn't going to get closed by the US government. At most if for some outlandish reason, transfers to and from Dwolla and bank accounts gets spanked, then this will be like poker websites, Suprnova.org, Napster, and the myriad of other examples where killing a centralized, popular "agent" of technology just increases the number of users drastically and creates 10 other entities that replace it.

Or transfers will just get slightly more annoying, such as there being another entity of some sort between Mt. Gox and people's bank accounts.

By the way, this is a US-centric discussion and Bitcoin is pretty darn diverse.