As stated, if you're going to run security/pen testing toolkits via the command line (and you don't intend to install Linux on the laptop), then anything with a decent amount of RAM, good CD read speed and a fairly decent processor will do. If you're going to install Linux on a laptop, find a laptop you like, and see if it is mentioned either on Linux-Laptop.net or Tuxmobil.org. Chances are it will be, and there you can find information on what worked or not. You can also look at the laptop's specifications and verify that the components are compatible by looking at your distro's HCL (eg. openSUSE 10.0's HCL can be found here).

Most laptops will run Linux w/ GUI fine - just try to get a laptop with an nVidia rather than an ATI graphics card, because if not you can run into some problems.. ATI's driver isn't the most "newbie friendly" one out there

Even if you do encounter problems, most can be resolved by adding a parameter when booting it, installing a driver, or whatever. I'm currently installing Gentoo on a Compaq Presario R3000T, and it's going fine atm - SUSE 10 worked great on it too. And I have SUSE 10.0 running on an Advent 7084 - admittedly, I haven't gotten round to installing the ATI drivers yet, but that will come in due course..

Cheers,

-jk