Just imagine if the Federal Reserve started their own cryptocurrency, created a FedChain, blessed the banks to use it, ect. I can't imagine the limitations on it, but I am sure they would be bad. Probably would be a way for the feds to create their own coins in the protocol.
Soon there would online exchanges like Cryptsy.com that would allow you to quickly switch your FedCoins to BitCoins.
I think ultimately, this would actually accelerate the fall of the dollar.
What has drawn a lot of people to cryptos is its decentralized nature. The Fed would just be in essence creating a digital USD. Don't think it would spark too much support.
If I were the Fed I would be working on creating cryptocurrency. It might mean dollars are used less but the average person would be much more open to using "Fedcoin" rather than bitcoin I would think. I don't think Joe Sixpack cares too much about decentralisation.