Marketcap as of 12/16/2013 (rounded): marketcap_BTCUSD = #coins * #BTC/USD = 12*10^6 * 800 = 9.6B$.
This is very roughly 1% of the money supply (M0) of the EU or US, or perhaps 0.05% of global money supply.
I am not sure if it is correct to compare marketcap vs money supply, because you cannot sell all bitcoins at stock price. Once you will start selling - it will go down fast.