"Jeff Garzik, a member of the Bitcoin core development team, says in an email that bitcoin is not as anonymous as the denizens of Silk Road would like to believe. He explains that because all Bitcoin transactions are recorded in a public log, though the identities of all the parties are anonymous, law enforcement could use sophisticated network analysis techniques to parse the transaction flow and track down individual Bitcoin users."
You don't even need "sophisticated techniques", you can follow the transactions in http://blockexplorer.com/
ALL transactions, including the receiver and sender addresses are public. They have to be, since the global bitcoin network has to verify and add them to the blockchain.
A full copy of a currency's block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each address at any point in history.
Basically, if the cops grab your box with your Bitcoin crypto keys on it, they know all of your transactions. This is why you need to encrypt your keys if you're doing something illegal.
tl;dr: Nothing ties you to your address but evidence not inside of the chain.
http://gawker.com/5805928/the-underground-website-where-you-...
"Jeff Garzik, a member of the Bitcoin core development team, says in an email that bitcoin is not as anonymous as the denizens of Silk Road would like to believe. He explains that because all Bitcoin transactions are recorded in a public log, though the identities of all the parties are anonymous, law enforcement could use sophisticated network analysis techniques to parse the transaction flow and track down individual Bitcoin users."