Knowledge about the Silk Road went mainstream earlier this month, thanks to a Gawker profile. But shutting the marketplace down could prove difficult, since it can only be reached via the distributed, anonymized network known as Tor. Furthermore, the marketplace uses a seemingly random assembly of letters and characters as its URL, which means that, should that domain name get shut down, its operators could simply open up shop under a different name, publicizing the new URL via underground channels.
That said, one weak point would be Bitcoin transactions, since they're the only form of currency currently accepted by the Silk Road. Bitcoins are decentralized currency, created in 2009 by Satoshi Nakamoto, who also released open source software which powers the decentralized peer-to-peer network that runs Bitcoins.
The weak point is bitcoin? Enforcement Fail.