Stick with SI prefixes.
10-9 is nano
So the smallest (current) unit would be 10 nano(bit)coins.
Edit: It seems helloworld posted the same thing, but more quickly. I agree with him.
As I described in this
post, using nano have it's implications.
Everyone used to see portraits of leaders on money. Why shouldn't we have personality attached to bitcoin as well?
Vote for Satoshi!