While I grant you that this is possible, it's probably risk #971 on my list of Bitcoin risks right now. China has already forbidden buying real things with virtual currencies, but I think it's very unlikely that many Western governments, especially the United States, will do so. It's almost certainly constitutionally protected, though it will trigger complex tax questions. I can't guess what will happen with China, but I don't think that it's a game breaker.
I think "fighting terrorism" in the form of preventing "money laundering" through Bitcoin will be more than enough justification to push a bill through Congress, and even if it is unconstitutional it won't really matter at that point, because Bitcoin will crash long before any constitutional challenge could possibly make it through the courts.
Just like alcohol use crashed with prohibition, or the drug market? Previous attempts to stop these have resulted in larger market shares of those willing to still do related business and almost always increases demand.
