and Windows(because it comes with it)
It doesn't have to. There are a lot of sites where you can specify exactly what you want and they will build it for you, so you choose weight, battery life, matte or gloss screen, os or no os, amount of usb ports, blah, blah, blah.... at a cost of course.
I think its very difficult to specify an ideal laptop for pentesting, its just a laptop with some pentesting software on it, so what you need to do is decide what is important to you (you already have in as much as you want it to be lightweight with a long battery life). You then need to factor in price and whatever else affects your buying choices and then continue as you would otherwise.
I have backtrack on a decent sized thumb drive and as such, whatever I plug it into becomes a pentesting environment. I also have a nice little samsung netbook which cost me under £200 which is very handy for going out and about and has never had any trouble doing anything I asked of it.
A wise man can learn more from a foolish question than a fool can learn from a wise answer.