So far, the Silk Road busts have nothing to do with Bitcoin. Everyone should assume that all BTC transactions are trivially traced, forever. Just make sure you aren't tying any identity you care about to dangerous transactions. That's the only (and of course difficult) trick.
If a drug buyer in a BTC transaction got the coins anonymously (say, sent a self-destructing robot to drop off cash in exchange for BTC), then that part is fine. They can go use those coins anonymously. If they ship drugs to their house, well... that alone sort of undoes it all.
The drug sellers that were caught from Silk Road were caught due to things like making a huge amount of trips to post offices, getting the attention of postal workers. That may be a parallel construction, but it seems legitimate enough. Buy drugs from large seller, look at the postmarks for patterns. Then go gumshoeing around and wait 'til you see the same car or people going to the same post offices over and over and over. You can probably pull this attack off without even having government capabilities.
If a drug buyer in a BTC transaction got the coins anonymously (say, sent a self-destructing robot to drop off cash in exchange for BTC), then that part is fine. They can go use those coins anonymously. If they ship drugs to their house, well... that alone sort of undoes it all.
The drug sellers that were caught from Silk Road were caught due to things like making a huge amount of trips to post offices, getting the attention of postal workers. That may be a parallel construction, but it seems legitimate enough. Buy drugs from large seller, look at the postmarks for patterns. Then go gumshoeing around and wait 'til you see the same car or people going to the same post offices over and over and over. You can probably pull this attack off without even having government capabilities.