a mini-itx mb doesn't need much power, actually. it's the peripherals, such as HDD (higher rpms demand more power), cd-rom drives, and optical drives, usb devices, which are power-hungry.
my mini-itx has a 150W psu, which is relatively large for mini-itx standards...which hover around 60-80Watts.
*edit: and therefore, a battery could, in fact power the puter, you would just have to pay a LOT of attention to what devices will be run, how much power you would need, and if the battery would viably run those peripherals. there ARE 2.0GHz processors in laptops, sucking a lot of power out...keep in mind that for the average builder, COST is the main barrier to a sucessful project, not "can it be done?" (end of edit*)
for modded mini-itx cases, most use external laptop-type psu's...for space considerations. ever notice on a laptop psu cord the large ac/dc inverter on the cord?
I agree though...building a true laptop would be a costly endeavor...not that it's impossible, just expensive and hard to find parts. most likely it would be a hybrid of old laptop parts, and new innards (mb, cpu, psu, ram)
~Branson