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
yes, there's so many poker websites currently functional for US customers. No funds frozen or anything.
also, unlike poker, the US isn't the only country that will likely fight back against what they see as a money-laundering scheme. I would expect that before too long (within a year) there will be some exchanges effectively shut down and possibly some other ones that just take the money and disappear into cyberspace.
Growing pains. That's all. Can't stop a peer-to-peer service.