Creating a sub network of approved bitcoins who's ownership/lineage could be verified. Perhaps by forcing companies to register their wallet addresses and the details of anyone buying through their bank using a credit card (this is assuming that the company doesn't want to break the law). A 3rd party (government) signed hash could be required for all transaction to prove legality.

Deep packet inspection could be used to locate people who send bitcoin transactions (by default transactions are far from anonymous). Offline seizure could be used to prove large wallets illegality (e.g. if they tracked down silk road owners). There are plenty of algorithms that can infer people you might know based on your FB friends friends, I'm sure they can be easily applied to transactions.