I'm somewhat concerned about this.

My (rather uninformed) impression is that if an innocent party receives property that turns out to be stolen, they have to return it, regardless of how many hands it has passed through.

The situation with money seems different, because money is fungible.  If someone receives money from many sources over time, one of which is from theft, and pays money to many others over time, who is to say who ended up with the stolen money?

Does it matter if the serial numbers of the stolen bills are known?  Suppose a thief steals some $100 bills, goes to a bank and exchanges them for $20 bills, and then vanishes.  Does the bank have to return those $100 bills because they are the stolen ones?  If the bank gives them to me as a withdrawal, do I have to return them?

It will be dangerous to accept bitcoins as payment if they become subject to "stolen property" treatment.  A large number of bitcoins may eventually have something shady in their past.  It's not a question for the bitcoin community to decide -- it's a legal question.