If memory serves me correctly, Apple's last sub-notebook was the Duo models. They haven't been around for many years now, at least 10. Below is the original sub-notebook by Apple and the docking station that could be purchased separately.
Now another sub-notebook is on the horizon and is rumored to be announced, no word on release date, at the MacWorld convention in January.
It is also rumored to be half as thick as current MacBook Pros, will use NAND Flash memory instead of a traditional hard drive and won't include an optical drive. The price point is also rumored (lots of rumoring here) to be somewhere between the MacBook and MacBook Pro models.
Erik