From The Cyclopedia of Law and Procedure.
http://books.google.com/books?id=7HMEAQAAIAAJ&pg=PA362 Stolen Property. A bona fide purchaser obtains no title against one from whom the goods were stolen, except in the case of negotiable instruments.
Federal Reserve Notes are negotiable instruments. Bitcoin is meant to pass like money but they are not promises to pay.
They aren't chattel either. They are more like an incorporeal hereditaments.
It's so new and original I don't think anything in present law encompasses them completely. However the fact that is meant to pass like money would incline the courts to treat it as a negotiable instrument. The reason negotiable instruments are excepted from the rule regarding stolen property was the "necessities of commerce and sound public policy".
That doesn't mean someone in possession of stolen goods that has notice of the good's tainted character gets a free pass. They can still be sued for it's value. Their recourse is to sue the person they received the stolen property from ....up the chain of transfers to the original thief.