0.1 BTC = 1 dBTC = 1 decibitcoin
0.01 BTC = 1 cBTC = 1 centibitcoin
0.001 BTC = 1 mBTC = 1 millibitcoin
0.0001 BTC = 100 μBTC = 100 microbitcoins
0.00001 BTC = 10 μBTC = 10 microbitcoins
0.000001 BTC = 1 μBTC = 1 microbitcoin
0.0000001 BTC = 100 nBTC = 100 nanobitcoins
0.00000001 BTC = 10 nBTC = 10 nanobitcoinshttp://en.wikipedia.org/wiki/Si_prefixes
I agree with using the SI system. I think 10 nBTC is fine, would have fitted better with nine decimals though.
Might be nice to give the smallest unit it's own name. I like the following suggestions:
microcent (since it's 1/100 [a cent] of a μBTC)
credit (standard currency in sci-fi)
bitcoin-atom (since they are indivisible)
bit (smallest digital unit)
Looking at https://secure.wikimedia.org/wiktionary/en/wiki/bit
bit is actually synonymous to coin
1 bit, 10 bits, 100 bits is 1 μBTC, 1 megabit is 1 cent and 100 megabit is 1 BTC. But maybe it's too confusing since bit is used frequently in other contexts.
My guess is that only SI prefixes will be used in the end if not, or as someone suggested, the decimal is removed entirely, so that 0.00000001 "old" btc = 1 "new" btc, it would be as easy as adding a multiplier in the software.