I'm curious about something. A hypothetical situation. If Ben was asleep while the coins were being sent to his wallet, which was stored in an encrypted file on his computer, and when he woke up, he couldn't remember the keys for decrypting that wallet file, would he still owe the sender the bitcoins?
I would say no.
In this case, I would argue that he never had possession, and thus never had an opportunity to return them to the rightful owner. By statute, and by common sense, in this case the theft wasn't the reception of the coins, but the willful failure to return them.
On the other hand, if they ever moved in the future, I would expect the defendant to end up back in court for theft, and possibly perjury. No problem with the statute of limitations, because again, the crime happens when the defendant has an opportunity to return them, but fails to do so.
It's called conversion (look it up on Wikipedia). He probably can't be sent to jail for it, but he can be sued and be found liable for damages, attorney's fees, and (if he can't return the BTC themselves and must reimburse in USD), damages can include any appreciation of the BTC. All he needs is an attorney to file a lawsuit.
Phantomcircuit, I would suggest you call his local police and file a police report, simply get a case number, make it clear that you simply want to file a report to document your complaint that this guy acknowledges having your property and is willfully depriving you of it and converting it for his own use. Whether you actually call a lawyer is up to you, but if you decide not to now and significant BTC price appreciation makes you change your mind later, you'll be glad you filed this report. Even if you do not live in the US, I am pretty sure you have standing to sue in a court in his local jurisdiction.
(My fingers crossed: hopefully an attorney who understands Bitcoin would take an interest in such a case, because it has the potential to result in a court affirming that Bitcoins have value - something that would be super good for Bitcoin)