From Satoshi / Malmi email archive

(…) 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.