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.
Just to clarify...the poll is specifically about their US based bank account(s). They have other banks accounts from what I understand. Also, I do believe their servers are located in TX at the moment. I think you are correct that such a problem would quickly get routed around (there are already other fully operational exchanges and mtgox itself is pretty international and I'm sure could adapt if something like this occurs).