My old Compaq Contura Aero 486/33 was called a "sub-notebook".

It literally was half the size of today's notebook computers.

It had great battery life and the form-factor was perfect for touch-typing from the seat-back tray of an airline seat.

What made it "special" was that it had an integrated thumb-ball on the right-hand side and the mouse buttons were on the right edge of the housing. Thus only right-handed people could use it.

But if you're squeezed into an economy-class airline seat with your elbows into your side you could still do work without too much trouble.

I think that the UMPC designation should only be for things that can fit in a standard size pants or shirt pocket.