So this all makes me wonder if there is a way to create a central database about fraudulent transactions, and associated addresses.
It's nearly impossible to mark certain Bitcoins as stolen or dirty because they can be so easily laundered. For example, send the stolen coins to an account at MyBitcoin.com, withdraw the coins to a new Bitcoin address. The withdrawn coins are completely clean and other MyBitcoin.com users end up with the "dirty" coins.
Though, realize if "bitcoincop" is a real "cop" then he may be thinking that is easy. Once you find one of those users, you question him, and when he tells you that he uses mybitcoin, then you go to mybitcoin and try to get them to release their records, afterall, they should be able to make the connection with the account that they were deposited into.
That said, if mybitcoin can be convinced (or compelled) to help, then this should be a trivial step. Of course, since you can access them as a location hidden service, and they require no real information to sign up, it could easily be a dead end too.... and that is before we even consider other possibilities.... like coin tumbler (or similar). Unless the thief was the only person using it at the time, and not particularly clever about it, simply going from one service like mybitcoin or mtgox to another, through coin tumbler with multiple addresses well... I hope you get the picture.
hell, I recall even seeing someone on Silk Road who was offering pre-laundered bitcoins for sale. They claim to do some sort of escrow, so its not even like that person could cheat and send back the same coins (not that it would be hard to determine, but as a scam, i bet would work most of the time) and wouldn't even know the buyers real name.... though, I guess if you were sure that he did it, again, its no better or worst than mybitcoin in terms of, you could at least ask him to help you pick the trail back up. (assuming that he keeps records)
Though, how you convince an anonymous people, running services intended to gaurd your anonymity, to voluntarily cooperate in compromising someones anonymity, even in an indeterminate way like this, is an open question. I guess its possible that accusations of thievery may sway them to help, but, they may want you to prove it before they are willing to help.
Afterall, its not like you can pull them into an interrogation room and get out the rubber hoses. That is, unless you can compromise their identities first.
