See this post from Ray Dillinger: https://bt.irlbtc.com/view/819656.msg9170781#msg9170781

I remember this discussion, actually.  

Finney, Satoshi, and I discussed how divisible a Bitcoin ought to be.  Satoshi had already more or less decided on a 50-coin per block payout with halving every so often to add up to a 21M coin supply.  Finney made the point that people should never need any currency division smaller than a US penny, and then somebody (I forget who) consulted some oracle somewhere like maybe Wikipedia and figured out what the entire world's M1 money supply at that time was.  

We debated for a while about which measure of money Bitcoin most closely approximated; but M2, M3, and so on are all for debt-based currencies, so I agreed with Finney that M1 was probably the best measure.  

21Million, times 10^8 subdivisions, meant that even if the whole word's money supply were replaced by the 21 million bitcoins the smallest unit (we weren't calling them Satoshis yet)  would still be worth a bit less than a penny, so no matter what happened -- even if the entire economy of planet earth were measured in Bitcoin -- it would never inconvenience people by being too large a unit for convenience.

Now that I think of it, we did talk about the floating point format in that discussion.  8-decimal divisibility was the maximum Satoshi would consider, for that reason (although he was a fanatic about doing everything with unsigned integers).   Hal's point about the smallest division being less than a penny, and that being possible even if the whole world's money supply were denominated in Bitcoin, meant no extraordinary measures were necessary.

When bitcoin was first launched, the client only showed two decimal places, not 8. Many users at the time did not know it was further subdivisible.

Here's another post from satoshi which might interest you: https://bt.irlbtc.com/view/44.msg267#msg267

Eventually at most only 21 million coins for 6.8 billion people in the world if it really gets huge.

But don't worry, there are another 6 decimal places that aren't shown, for a total of 8 decimal places internally.  It shows 1.00 but internally it's 1.00000000.  If there's massive deflation in the future, the software could show more decimal places.

If it gets tiresome working with small numbers, we could change where the display shows the decimal point.  Same amount of money, just different convention for where the ","'s and "."'s go.  e.g. moving the decimal place 3 places would mean if you had 1.00000 before, now it shows it as 1,000.00.