[sp332]

Think about it as cash: if someone knows that serial #AAAA was stolen, and then later you deposit that bill at a bank, they still have no idea if you are the person who stole it. It could have gone through any number of hands. I think I see your point about mixers though.

[Groxx]

It couldn't have gone through any number of hands - Bitcoin transactions are public. You can see every hand it has gone through, though you may not (or may!) know the owner of the hand. That's why any large-scale analysis can be dangerous to such actions - if you discover the intermediaries, you can start drawing conclusions about who performed actions.

edit: here's the main article I'm aware of http://anonymity-in-bitcoin.blogspot.com/2011/07/bitcoin-is-...