Blind signature mixing. Give someone running OT 10 bitcoins, you get 10 blind signatures from them which pretty much say "I owe the holder of this 1 bitcoin, to be redeemed by the first person to present to me". Because it is blind signature, the signer can confirm their signature and so can others but when the signer signs Alices certificate he can not see what his signature looks like so he can't link it to his transaction with Alice when it is cashed in. So if 1,000 people use the same service, then strong unlinkability is obtained, although care must be taken to avoid correlation attacks so open random accounts and don't cash in and out X:X or the link can be assumed. OpenTransactions compliments Bitcoin a lot and I hope more people start running it because unlinkability is a service a lot of people need and would be willing to pay some small % for you just to run a server they can mix through.