@12" users (me too formerly): For a long time there was nothing comparable with the powerbook, considering its optical audio in, dvi, gpu, and fast processor. However, Asus just released the WXGA 12" U6S w/ nvidia 8400 & HDMI @3.5lbs which is exactly what the powerbook 12" with a bit better batter life at possibly weighing a bit more 3.9lbs (also to avoid competing with MBA).
A 12" MBP would be cheap and easy to produce because the ODMs already have similar models.
The chips and the tech are there to build smaller machines--But I wonder if Apple really sees itself as a core computer business anymore (they changed their name after all).
They don't need a zillion models like asus, dell or Hp, but maybe etending the line to 5 (with 12" MBP) wouldn't be a bad idea to cover each weight class from 3-7lbs.