IRS Now Has a Tool to Unmask Bitcoin Tax Cheats: http://www.thedailybeast.com/irs-now-has-a-tool-to-unmask-bitcoin-tax-cheats

...Chainalysis, a startup offering its “Reactor” tool to visualize, track, and analyze bitcoin transactions. Chainalysis’ users include law enforcement agencies, banks, and regulatory entities.

The software can follow bitcoin as it moves from one wallet to another, and eventually to an exchange where the bitcoin user will likely cash out into dollars or another currency. This is the point law enforcement could issue a subpoena to the exchange and figure out who is really behind the bitcoin.