Be weary of the relatively new form factors like pico-itx.
The Pc-104 form factor is the de-facto standard for computerized robotics (and has been for a while). ( I work for the DoD and I can't think of a robotic platform that isn't PC-104). More then a small size, the standard is 'stackable' and scalable; with a whole market of add-on boards to suite any need. Need more com ports? just get an expansion card with 12.
I won't get into an OS war, (zs loves those :-P) but I would recommend linux as a robotic OS, and suggest you install a real-time kernel.