I'm wish I had the cash for an Asus EEE. It's close to the size of a DVD case, 7" screen (I think) with 800x480 res. Has all the other goodies you'd expect in a full size laptop minus an optical drive. That, and instead of a platter hard drive it has a 4gb solid state drive (flash based hard drive). It's $400.
But, if you really want a powerful computer that can run an OS, but is small enough to strap to a bot, check out gumstix. Tiny!