When the US Government gets involved and starts regulating BTC to the point it is very hard to get, or it is easier to just use the USD Bitcoin will tank and fast. If the US government wants to shut down BTC they will do it just like they did with the poker sites and stop the flow of money.
What, exactly will they regulate out of existence?
Trade in:
-Bitcoin specifically? That's a nice game of whack-a-mole they'd be setting themselves up for.
-Virtual currencies in general? There are a lot of them, I'd rather not even try to figure out where the unintended consequences of such a move would lead. Say goodbye to your MMOs, for one, if this comes to pass.
The argument I'm trying to make was made much better here:
http://www.finextra.com/Community/fullblog.aspx?blogid=8019
The problem with that is if you want to outlaw all virtual currencies you have to make the laws broad enough to encompass any new configuration of a virtual currency that might arise. If you make it broad enough to accomplish that goal then you could very well end up inadvertently outlawing all non-local currencies, because at a broad level Bitcoin is indistinguishable from a real-world currency (as Judge Mazzant rightly pointed out). However, you would also make illegal more ‘legitimate’ virtual currencies such as Mint Chip, which is being incubated by the Canadian Mint currently. You’d probably end up making airline miles, zynga coins, and other such variations on the currency theme also illegal. The upshot is that Bitcoin and all other virtual currencies or pseudo currencies cannot be made broadly illegal simply by virtue of the fact they are virtual, and current laws that define currency as a physical commodity or a financial instrument as written are hopelessly out of date and are essentially aiding the proliferation of Bitcoin.