I'm a convert to Apple laptops. They're a bit more expensive, but they're reliable, the battery life is decent (I get about three and a half hours off a full charge if I just turn the backlight down on my PowerBook), and Apple's support is second to none. The only other company that will come close in support is IBM.
The ThinkPad laptops are also very nice, but most of them are rather clunky. The only exception is the (IIRC) X-series, but I've only personally used the T- and R-series.
Dells I think are underpowered and overpriced, and their support really sucks. Being a computer science student, I get requests to "fix" those laptops the most.
I've seen some nice laptops around from Toshiba, but haven't personally worked much with them.
One of my friends just got an Acer laptop for under $500. It works just fine, as long as you don't try to do more than one thing at a time. Which means, don't try to write a paper and read research database results at the same time... so not the greatest for school work.
