I bought my netbook specifically for this, given that I'm often on my motorcycle.
I have a mobile computer repair business myself.
You're definitely going to want some cabling to be able to run IDE/SATA drives on your computer via USB, which is very helpful when it comes to spyware problems, or quickly retrieving data the client thought to be lost.
Also, small non-magnetic tools of course.
Boot discs
I carry around Ubuntu on a flash drive as well, and the majority of the software packages I recommend to clients, on another.
Always carry extra cables, USB, VGA, and Cat5.
This here is a good collection of free software.
http://www.runtime.org/data-recovery-downloads.htm
For me, nothing much else has been required.