I apparently do a lot more editing in the field than most do. I do the usual quick culling and more critical selecting of images. I find the sooner I do it, the more familiar I am with the dive. That helps not only with selection but the feedback loop that helps me learn. I'd say that what Alex says makes complete sense but since I don't dive as often as he does I never cull as ruthlessly. My goal is to bring back a collection of images to show---if it's a crappy trip then it's a crappy collection. A pro's goals are different.
I also perform the bulk of my conversions on location. Most will say that they want a big monitor for that but that's why I bring a 17" machine. My goal in the initial conversion is a neutral, linear rendering with correct exposure, black and white points. That doesn't take a big monitor or a large gamut. For the most part, what it takes is some practice and a good histogram. It has to be a pretty bad notebook not to be able to do this job.
When it comes to creative editing, obviously a better workstation is valuable, but with Aperture, Lightroom, and Lightzone performing these tasks nondestructively there's no reason NOT to perform them even if they have to be adjusted when you get home. No job is complete until you've looked at them carefully on a system you trust. You will make changes but what does that cost you?
I've found that most times I have left Lightroom for Photoshop on the road that I've been unhappy with the result. The reasons for doing that are becoming fewer, but I think in those cases you really need the right monitor to help you. You can edit on the road up to completion or to the point where a decision you make isn't easily undone.