Intel has recognized the general confusion about MIDs versus netbooks versus notebooks. The prime differentiation appears to be screen size and portability. MIDs are recommended for 4.5"-7" screens, netbook 7-10.2", and notebooks 12+". Associated with the screen size is the general device size. The processors change as well, although both are targeting the Intel Atom. The Atom processor comes in two physical sizes, the Z series for MIDs and the N series for netbooks, with the Z series being much smaller and somewhat slower.
The confusion has been growing for a while and I’m not sure that defining devices by screen size is really going to clarify anything for the majority of users.