I'm obviously missing something, what does "money supply" mean other than the total amount of Bitcoins in existance?
there will only be 21 mil. REAL btc. but you can still OVER lend them. and thereby creating virtual BTC.
eg. i put 100 btc in a bank. the bank lends someone 50 btc. so now i having 100 in the bank and someone is having 50 of my btc. 100+50 = 150.
as long as all the btc is not needed at once it should be relatively safe.