The US will criminalize bitcoin, it's only a matter of time.  The more interesting impact is how much effect they will have on partner countries to criminalize it as well.  Given the amount of influence the US currently wields in the banking industry, this is likely to be large.

An interesting thought experiment is how large of an economy Bitcoin would have to be before US officials take notice.   Before e-gold was shut down, it has ~$600 million in their accounts.  In order for Bitcoin to become a similarly sized threat each coin would have to be valued at $82US.