How can they? It would have to be pretty creative 'cause you can't really bring down the network.

Perhaps if they invested in huge amounts of computing power to get over 50% of the hashing they could case some problems, but since our hasing power is something like 40x the worlds biggest supercomputer they would really have to sped big time. They could try to crash the market with a big influx and outflux of cash and i guess there is a chance that could happen but I think in either case it would become quite apparent very quickly and we would have a good chance of stopping it.

One route they might go down is to prevent trading by shutting sites like MTGox. We need to decentralise our trading urgently. If MtGox went down it would cause pandemonium.

In the end if we keep going for a just a little while longer i think the bitcoin userbase will be so big and so diverse that one government acting alone would stand very little chance.