I see this closely mirroring the whole music piracy phenomenon ten years ago. It starts as one common place that anyone can use freely. (Think Napster.) Then it's discovered and condemned by the media and the law, and gets inundated by many more people due to the advertising, both people for and people against its purpose. Soon it shuts down, but then more open up. At this point it becomes a game of being hip enough to know which drug-trading sites are new enough to be under the radar, but old enough to have a group of sellers with established reputations. (Think Audio Galaxy.) That's the tricky part. The way I see this playing out in the end is we'll end up with an internet full of gated communities for buying and selling drugs, each one with its own tortuous set of hoops that need to be jumped through in order to gain membership, and most people not qualified for membership in most. (Think Demonoid.)
As for the bitcoin, I see this technology being able to dodge currency laws by changing its wording and coding a bit, to expolit loopholes that make the currency appear to be something along the lines of a casino chip, MMORPG gold, or some other kind of game token. At the very least an insincere disclaimer of 'no cash value', with the inability to convert bitcoins back into any real currency, is in order. Regardless, I could forsee a black market of currency speculators/investors springing up, who don't use bitcoins to buy anything but real currency, and are able to make a mint, say, buying bitcoins with yen, selling them on craigslist a week later in dollars, converting the dollars back to yen, and reaping a profit. I'm no economist, but the complete lack of regulation of a currency can cut both ways. Namely, its value could fluctuate wildly and unpredictably in short periods of time, without any sort of buffer. I sure wouldn't buy any large quantity of bitcoins.
If legislation is drafted in the wake of all this, I see it taking the form of having a computer in your possession with any traceable evidence of having visited Silk Road or its ilk, being an punishable offense. They might even make it illegal to have a computer with any bitcoins on it. This will probably be written in legal language that's purposely meant to be broadly interpreted, just like the Analogue Act, to cover any and all future online drug markets. You order drugs from someone who turns out to be a DEA undercover, with positive reviews from shills who are also DEA. They have your address. Now all they need to do is show up at your door and seize your computer. They don't even need to find or look for drugs.