Cash exists. It's anonymous. Decentralised (not issueing, but existence, as it is dispatched everywhere). And yet the government seems Ok with it.
So they could use bitcoin to issue "electronic cash".
Basically they would modify the bitcoin source code, and make a new block chain with all the 21 millions "FEDcoins" generated all at once, in the genesis block. They could also make it so that a special private key could generate any number of new coins anytime they want.
And then they would guarantee the convertibiliy between FEDcoins and Federal Reserve Notes.
FEDcoins would be just like FDR notes, except that they are designed to be exchanged on the net, and not in person-to-person physical exchange.
That would cost them almost nothing.
They already have this, it's called FedWIRE.http://en.wikipedia.org/wiki/Fedwire