Re: This is interesting
The Bitcoin protocol does not have any notion of "stolen", and nor does it have any central authority that could label coins as such. (Such an authority would have enormous power, and power corrupts; Bitcoin was invented as a distributed mechanism precisely to avoid such centralised power). Hence coins which one person claims are stolen, can still be freely traded under the Bitcoin protocol.
In theory you might try to reclaim stolen coins using conventional legal means. That would require identifying who currently possesses the coins so they can be reclaimed. That can be tricky because the coins are stored in (effectively) a numbered account and there is no registry that maps account numbers to real-world names and addresses. So for example, if the thief uses stolen coins to order a pizza, you might be able to discover the address the pizza was delivered to. That would require the cooperation of the pizza vendor; and if the thief holds onto the coins for 10 years without spending them there is little you can do for that long. Alternatively, you might be able to discover the IP address the coins were acquired through. Generally, there are things you can try, but there are also things the thief can do to defeat you. Identifying a Bitcoin thief requires technical know-how and resources (or a stupid thief).
In addition to technical challenges, you would have to prove the coins were stolen in some court that has jurisdiction. Lots of legal issues there. Arguably if the thief did not have good legal title to the coins, they couldn't pass good title to the pizza vendor, and you could reclaim the coins from him (much as you could reclaim a stolen car even if the current possessor had bought it in good faith; it would still be your car and your bitcoins). However, it's not clear Bitcoin works like that; cash doesn't, and Bitcoin was intended to work as cash, with transactions being irreversible.
Some of the sums involved in recent thefts are large enough that I would expect all legal and technical avenues to be pursued in recovering them. So maybe MtGox will get its coins back. It won't be quick, though.