From the 'netbooks are a real platform' files:

Netbook or Mobile Internet Device (MID) makers like Asus and Acer use various flavors of Linux as the underpinning of their devices. The idea is that you can tweak Linux to get a minimalist JeOS (just enough operating system) to provide users with what they need.

Now Ubuntu is jumping with both feet into the fray with Ubuntu MID Edition which is specifically built for the MID category of sub notebooks.

"We believe that Ubuntu MID will be used by a range of people and companies," David Mandala - Project Manager of the Ubuntu Mobile and Embedded Group wrote in a blog posting. "The most typical case will be purchasers of a MID device who use it as the installed OS with the shipped applications. Users might install additional applications which will be created within the Ubuntu Mobile and Embedded community and in Moblin.org"

Mandala also noted that the MID version will now become part of the regular release cycle for Ubuntu - meaning that Ubuntu is treating MID as a serious category.

Frankly on the small (and older low power) devices I've used Ubuntu on - I've generally used Xubuntu which provides the Xfce GUI which is a whole lot smaller and less memory/CPU intensive than GNOME which is what Ubuntu generally uses. Still the Ubuntu MID effort is a great idea as it will leverage the great resources available in the Ubuntu community for MID devices.
 
Of course Ubuntu is not alone in supporting MID, Novell also is very active in this area - though to date I haven't seen Red Hat focus on it specifically (yet).