In the reference client it's represented as a whole number and infact the denomination Bitcoin is an abstraction on top. They may have changed this recently but I was under the impression the units are not represented as a decimal number.
That's correct, Bitcoins are represented by 64-bit integers, since it's usually a very, very bad idea to use floats in the financial realm.
I suspect that Bitcoin's lack of divisibility past the satoshi won't ever be an issue, since there will probably be other digital currencies in use for small, everyday purchases (perhaps one of the altcoins, but more likely one that hasn't yet been created).
