While I agree that a bitcoin central bank is completely against what bitcoin stands for, I think the idea of being able to opt for a full reserve or fractional reserve account at a bitcoin exchange is pretty good. Users could choose to put their coins to work and get some interest in return or just keep them in storage without interest. It could possibly help bitcoins liquidity problem too. This might be irrelevant though if places like btcjam and other ways to loan money become more prevalent.