There are several different mobile pentium processors you can find in intel laptops: P4-M or Mobile P4, Centrino, Desktop P4 (hard to find), et cetera.

If battery life is that important I would recommend as others have said, two batteries or having an additional spare to swap out when one dies.

In terms of processing power, anything will do fine, P3, centrino, P4, as long as you are not planning on gaming, I would recommend a P3 or a Centrino. If you are planning on gaming on the laptop as well, I would recommend hauling around two additional car batteries to get 5 hours.

In terms of battery life for processing power, the lower Mhz the better. I have noticed from my personal experience that the centrino configuration of notebooks works quite well. They have the ability to change the speed of the processor on the fly to throttle it back when it is not being used as intensively.

To maximize the amount of battery time you get out of your system, I would not recommend putting 512MB of ram into it, unless you have a specific reason that makes you need to do so. Hard drive size does not make too big of a difference, just make sure that it is a 2MB cache drive that only has a spindle speed of 5400 RPMs.

Screen size will also make an impact in battery life. Smaller screen will give you more battery life. Also lower resolution will make a small impact in battery life as well, so go for the lowest resolution acceptable to you. SVGA will most likely be the lowest you will be able to find today.

There are several things that will eat up your battery life once you get your laptop. Any type of 'high-end' image or video editing software that is cpu intensive, as well as any type of new first person shooter game or the like. Viewing dvd's from a disc will also use quite a bit more battery than just casual e-mail and word processor usage.

One other thing to take into consideration will be the power of the wireless card that you will be using. If you have the dB transmitting power of the card turned all the way up or are maxing out the bandwidth (transfering files, downloading from a fast internet connection) you will also notice a significant drop in battery life time.