We can get an idea of what to expect by looking at the history of e-gold. The US government started attacking the exchange sites before they went after e-gold itself, and the banks would deny them accounts, making it increasingly difficult for people to exchange dollars (or whatever) for e-gold. We can certainly expect them to do the same thing with bitcoin exchanges eventually, because they already know it works.
I don't really have an answer to this. While it's nice that Intersango exists because competition is good, it's apparently located in the UK, which is just as subject to the US government as Japan is. Maybe an exchange will crop up that's just as (relatively) easy to get US dollars to and from but is located in a country where the government has both a backbone and a nuclear arsenal, but unless that happens I think we're pretty much enjoying the high point of national currency-to-bitcoin exchange right now.
The good news is that e-gold did hold on after the exchanges started getting shut down--it wasn't until the US government attacked them directly that they went down. With bitcoin there is no central authority, making it much harder for the government to attack. With major economies in the world circling the drain, the demand for bitcoin (or something like it) seems likely to increase, hopefully fast enough to make it impossible to do an attack by controlling the majority of the network.