But even with that, I'm only finding statistics for the US. In order to answer the OP's original question, we need to know the value of the world's entire money supply.
ETA: Okay, I found an estimate of world M3 at $75 trillion, which, if it's at all accurate, makes my numbers not horrifically far off.
ETA:
http://gizmodo.com/5995301/how-much-money-is-there-on-earthFor the narrowest definition, or what's called "M0", that includes only physical money, paper bills and metal coins that constitute currency. That figure is around 5 trillion dollars. The next step up is M1, which includes all the physical money, plus quickly accessed money like that in checking accounts, and comes in at $25 trillion. M2 includes M0 and M1, but also pulls in stuff like savings accounts and CDs under $100,000. That figure is around $60 trillion. And the last figure, the $75 trillion M3, is much more abstract and not often cited in official figures. It includes institutional money market funds, long term deposits, and other stuff rich people possess that can somehow be spent but confuses the rest of us.
ETA:
But actually, I think M1 or M2 is a better measure for this than M3.
Using M1: If M1 =~ $2.5e13, then, at 100%
BTC, 1
BTC =~ $1.2 million
Using M2: If M2 =~ $6e13, then, at 100%
BTC, 1
BTC =~ $2.85 million