[hugh3]

I imagine they'd make it illegal to accept bitcoins as payment, which would remove all legitimate users from the market, but illegal drug buyers and sellers may still use it as payment, along with underground bitcoin/cash currency exchanges.

Sure, but now you've gotta participate in two illegal operations instead of only one. Instead of exchanging cash for drugs you're now exchanging cash for bitcoins and bitcoins for drugs. You've doubled your chances of getting caught. And unlike the drug dealer, the cash-to-bitcoin converter doesn't have much of a profit margin, so what the hell is his incentive for staying in this illegal business?

What would an underground bitcoin/cash currency exchange look like anyway? A smoky room? There's no way to anonymously and untraceably recieve cash on the internet, is there?