Bitcoins relative lack of privacy creates a problem with tainted coins
risking becoming unspendable, or spendable only with some users, or at a
discount. So while the policy coded says all coins are equally acceptable,
the information exists so people can unilaterally reject them, depending on
what the taint is. So far revocability hasnt reared it's head that I heard,
nor taint inspection too much? However people have the choice and technical
means to check the taint and send the bitcoins back.
I'm not convinced that this will become a problem, socially. The same problem exists with real-world cash, which has serial numbers. The technical means to track dollar bills exists. But in practice, nobody bothers. It's prevented by law, but even if the law didn't exist, I'm not sure that mainstream people would care.