If the government were to invest a billion dollars in ASICs, is the system at risk to 51% attack? Perhaps yes. What to do?
we switch to scrypt and 1 billion dollars worth of government money poofs into thin air.
And? 1 billion dollars is how expensive when talking about government? According to 2011 numbers, the US government spends $10.4 billion per day. Do you think this number has gone down in the past two years?
The point stands, however, that it would not be very hard to make necessary decisions and changes to counter a >51% attack by anyone. Bitcoin might take a hit, but it'll develop scar tissue and become all the stronger in the long run.