A setup where you know the address you send money to... but where no one can determine where the funds in a given address came from.
And how would that be possible in a decentralized way?
The only setup I'm aware of that could do what you say is e-cash (blind signatures, part of Open Transactions). But this algorithm requires a server to perform the settlement. Perhaps the servers could evolve to multiple hidden servers where very few trust is needed. If you could easily move money from one e-cash server to another (can we?), and if you could be sure they can't steal from you (multi-signatures!), then we could practically have an e-cash decentralized network. Perhaps it's time to study Open Transactions...