Each Bitcoin can be split into 100 million parts, in addition there is a possibility of the division being pushed further if ever needed (the smallest unit could be split down more if the people accepted this change)
Also there is talk of making the default traded unit a mBTC, (I think) Its a unit that represents a thousandth of a unit. once this becomes the standard unit the psychology is there to allow BTC to rise even further.
Or is it a single unit or nothing?