The whole public-ledger setup of Bitcoin can lead to a technological arms race on mixing/tainting. Of course, tainting only works if it's done on an international level as otherwise there'll be a p2p market for swapping US-tainted coins for Russian-tainted coins.

Anyway, sounds like no-one is really interested in this. I suspected as such but I thought I'd throw it out there anyway.

EDIT: Having thought about coin-swapping a bit more, it's a pretty complicated problem with a lot of attacks so I don't think it would immediately kill off more local tainting.