Should we refer to this upcoming event as the centening?

Although Satoshi compares the price of Bitcoin to the Euro in his email, some simple mathematics indicates he may have had a much grander vision for Bitcoin—better explaining why the 21 million maximum figure was chosen.

At the time of Bitcoin's creation, the entire world's money supply stood at approximately $21 trillion. This figure, known as the M1 money supply, is made up of the total value of all the physical money in the world, including cash, coins, travelers' checks, and more.

If Bitcoin were to grow to become the single world currency—replacing all those that the M1 figure is comprised of—then each BTC would be worth $1 million. Because there are 100 million satoshi in each Bitcoin, this would place the value of each satoshi at $0.01.

My choice for the number of coins and distribution schedule was an educated guess.  It was a difficult choice, because once the network is going it's locked in and we're stuck with it.  I wanted to pick something that would make prices similar to existing currencies, but without knowing the future, that's very hard.  I ended up picking something in the middle.  If Bitcoin remains a small niche, it'll be worth less per unit than existing currencies.  If you imagine it being used for some fraction of world commerce, then there's only going to be 21 million coins for the whole world, so it would be worth much more per unit.  Values are 64-bit integers with 8 decimal places, so 1 coin is represented internally as 100000000.  There's plenty of granularity if typical prices become small.  For example, if 0.001 is worth 1 Euro, then it might be easier to change where the decimal point is displayed, so if you had 1 Bitcoin it's now displayed as 1000, and 0.001 is displayed as 1.