What would be an estimate of the value gained if the whole world started using Bitcoins?
According to xkcd: Money Chart estimated total economic production of the human race as of 2011 is $2,396,950,000,000,000 . Divide it by 21 million and the theoretical maximum price per bitcoin would be $114,140,476, but that figure is impossible, because for one, even in best case scenario Bitcoin will co-exist and compete with other currencies and would have only a small share of the pie. Second, Bitcoin is more efficient medium of exchange then USD, in other words, to complete transaction a someone required to hold bitcoins for shorter period of time therefore less total amount of bitcoins would be required for circulation and that also means lower price.