The less totalitarian governments (I include the US here) have no immediate plans to do any crazy-ass confiscations. But, if and when they decide to borrow a page from FDR ... it will be too late. Bitcoin will be entrenched.
The head of the USA Treasury department that oversees FinCEN has said in recent interview (have a link to it on my thread) they are monitoring adoption very closely. He specifically said that government oversight will increase if it becomes possible to move large amounts of money with Bitcoin and/or you can live by spending only Bitcoin. And that was only the financial crimes division. He said other departments were watching it with other mandates. This will not fly past their radar. He specifically mentioned anonymity as a threat many times. He also said they have people tracking the block chain.