Researchers from University College Dublin attacked the issue by analysing a theft of about half a million dollars worth of Bitcoins this June. They zeroed in on the thief's network, his relation to the user, and his relation to other publicly-known accounts. From that, they determined that by mapping the movement of funds and his association of users, they were able to determine the identity of not just the thief, but large swaths of his Bitcoin network.
Practically, this doesn't mean much if you're buying $US15 worth of weed on the internet. But for wider applications of the technology, like donations or payments to rogue organisations like WikiLeaks, you should know that your veil of secrecy only exists so long as no one goes looking for you. [Clique Research Cluster via Slashdot]