Up until about 1999 the laptop market was completely crazy. The technology was in a state of violent flux, with ``standards'' phasing in and out and prices dropping like rocks. Things are beginning to settle out a bit more now.
One sign of this change is that there are now a couple of laptop lines that are clear best-of-breeds for reasons having as much to do with good industrial design and ergonomics as the technical details of the processor and display.
In lightweight machines, I'm a big fan of the Sony VAIO line. I owned one from early 1999 until it physically disintegrated under the rigors of travel in late 2000, and could hardly imagine switching. They weigh 3.5 pounds, give you an honest 3 hours of life per detachable battery pack, have a very nice 1024x768 display, and are just plain pretty. Their only serious drawback is that they're not rugged, and often fall apart after a year or so of use.
If you want a full-power laptop that can compete with or replace your desktop machine, the IBM ThinkPad line is the bomb. Capable, rugged, and nicely designed (though somewhat heavyweight for my taste). I now use a ThinkPad X20, the lightest and smallest machine in the line, and like it a lot.
These machines are not cheap, though. If you're trying to save money by buying a no-name laptop, here are things to look for:
First: despite what you may believe, the most important aspect of any laptop is not the CPU, or the disk, or the memory, or the screen, or the battery capacity. It's the keyboard feel, since unlike in a PC, you cannot throw the keyboard away and replace it with another one unless you replace the whole computer. Never buy any laptop that you have not typed on for a couple hours. Trying a keyboard for a few minutes is not enough. Keyboards have very subtle properties that can still affect whether they mess up your wrists.
A standard desktop keyboard has keycaps 19mm across with 7.55mm between them. If you plot frequency of typing errors against keycap size, it turns out there's a sharp knee in the curve at 17.8 millimeters. Beware of ``kneetop'' and ``palmtop'' machines, which squeeze the keycaps a lot tighter and typically don't have enough oomph for Unix anyway; you're best off with the "notebook" class machines that have full-sized keys.
Second: be careful that your laptop meets the minimum core and disk requirements for the Unix you want to run. This is generally not a problem with desktop machines, which can be upgraded cheaply and easily, but laptops often have more stringent constraints.
Third: with present flatscreens, 1024x768 color is the best you're going to do (though that may change soon). If you want more than that (for X, for example) you have to either fall back to a desktop or make sure there's an external-monitor port on the laptop (and many laptops won't support higher resolution than the flatscreen's).
Fourth: look for Nickel-Metal-Hydride (NiMH) batteries, as opposed to the older (Nickel-Cadmium) NiCad type. NiMH batteries are great because they have considerably higher energy capacity per pound that NiCads. They need special circuitry to charge them fast, so don't try to throw out your NiCads and replace them with NiMH cells if you use a fast charger intended for NiCads. Both kinds of cells can be damaged by overcharging at rates faster than 10 hours per full charge.
Fifth: Older laptop electronics were 5-volt CMOS. Most current designs are 3.3-volt CMOS with power-management features on the processor (these are often labelled APM, Advanced Power Management). Buy this, if you can, to nearly double your use time between recharges.
Sixth: about those vendor-supplied time-between-recharge figures; don't believe them. They collect those from a totally quiescent machine, sometimes with the screen or hard disk turned off. Under DOS, you'd be lucky to get half the endurance they quote; under Unix, which hits the disk more often, it may be less yet. Figures from magazine reviews are more reliable.
Seventh: You probably want a color dual-scan display. It used to be that you had to choose between passive-matrix LCD (cheap, miserable color) and active- matrix LCD (great color, horribly expensive). Dual-scan passive-matrix is nearly as good as active-matrix, except for the narrower viewing angle, and it's much cheaper. Avoid the older single-scan models, sometimes marketed as having STN (super-twisted nematic) displays.
Eighth: get either a CD-ROM drive or an Ethernet card. Otherwise initial load of your Unix could turn into a serious problem...