What is the maximal value of 1 BTC? If all of the world suddenly decided they want to use BTC as their currency, what would the value of 1 BTC be, as measured in today's US dollars?

(Of course dollars would drop to zero in this theoretical case, but we can still measure BTC in today's dollars and get a finite result)
Basing on this set of data from XKCD, world's GDP is $62,900,000,000,000, If all of it was expressed in 21 million bitcoins, then each bitcoin would be worth $2,995,238.10, so roughly 3 million dollars.