Assuming it reaches pretty widespread use, an interesting (if extremely back-of-the-envelope) number might be the world's money supply divided by peak bitcoin (21 million). That is to say, if all the world's money were replaced by bitcoin right now, that's about how much value each bitcoin would have to cover.

I'm to lazy to track this down for the world, but a quick search indicates that the US M2 money supply is about 10.5 trillion. Replacing that with bitcoin would require each bitcoin to be worth $500,000.