I would think that online poker players or other gamblers figured this out a while back.
Back to the article, like another poster said, how is a bitcoin-based drug site any different in terms of anonymity than a cash-based one if you have to give your address out at the end anyway?
The idea, in theory, is that the use of BitCoins cannot be tracked by a third party. That is to say, Silk Road, you and your dealer are the only ones who know about the purchase. In theory all three of you do not keep records of the sale, unlike credit card companies, which keep records of EVERYTHING.