JoelKatz: Okay, so someone buys something from me using coins who were a few hops down the line was scammed or stolen from someone. Keep in mind that at every step of the way no one gets a choice of what coins they receive, which they spend, etc. As stated previously, pick any of the richest BTC addresses and send some scammed coins there - now what happens?
The guy who stole 75000 bitcoins from mybitcoin.com is not going to spend them inside the bitcoin economy; he is much more likely to convert them to fiat. This is because in the world we live in, where bitcoin has still not taken over, inferior currencies such as the dollar or euro are still being used by most people, and are still useful, no matter how inferior these currencies are :-).
This means that the "someone buys something from me using stolen bitcoins" scenario will most likely be "someone receives partially stolen bitcoins from an exchange". When this happens, if the receiver contacts the victim, the victim can forward the bitcoin addresses to the police, and the exchange will receive a court order. With a court order, the exchange will have to provide information on how the money has been laundered, and on which bank account it ended up. Money can be tracked in the real world, perhaps not as easily as in the blockchain, but it can be tracked at least by interpol.
If this could lead to the recovery of the money cashed out by the scammer, I am sure Bruce Wagner would be very happy and smile again, even if he would recover fiat money instead of bitcoins. More important, catching a thief with the help of the blockchain would have a tremendous impact in the media, and it would dramatically change the image that the general public has of Bitcoin. Such a feat would certainly help exchanges to get to do businesses with banks, without having their accounts frozen on suspicion of mauney laundering.
