Some factors for you to consider in making your decision.
Heat: one of the costs of making today's laptops and netbooks skinny and sleek is that there's no space inside for air circulation. A few months ago, I bought a used HP G70, because it cost only a bit more than a comparable desktop. I planned to use it to replace my slowly-dying desktop PC, while also having something powerful and portable whenI have to do work on the client's site.
Well, spring has come, and the days are heating up in the desert. Yesterday, I came home from work to discover that the HP had shut itself down due to overheating. This despite the fact that it sits on a wire shelf with free air circulation all around it. So much for replacing my 24/7 desktop system...
Today's mania for ridiculous levels of speed and graphics power means that even slightly-slower used machines are often available dirt cheap. If you're mostly going to be using Linux, that "ancient" XP machine can be a real speed demon for you. And you may find it improves your programming self-discipline if a recompile takes 5 seconds, instead of 3 
I've got a friend who does IT for a chain of car dealerships. Nearly all their mechanics have company netbooks for interfacing to the diagnostic equipment and the company's management system. He's settled on Dells, after trying several others, because the mechanics don't seem to break them as easily. It's only one data point, but it's a pretty good one.
If you do go with a netbook, even though your vision is probably a lot better than mine, you'll probably find yourself wanting to hook it up to a monitor for long work sessions. The HP Mini that I got almost free due to SSD problems (easily circumvented because Linux isn't stupid about managing bad areas) turned out to be not so much of a bargain: it uses a proprietary video cable that's been discontinued, and only available from people who scarfed up the last ones and are selling them for 4 or 5 times the original price. The bleeping cable costs almost as much as a used netbook 
And make sure that the netbook doesn't just give you a bigger 1024x600 display when hooked to a big monitor.