With the many choices and factors to consider, choosing a laptop of any kind can be a considerable challenge. Choosing one for use with Linux, however, brings its own special set of considerations, since it's not yet always a plug-and-play world for the open source operating system. Here are some guidelines for choosing the one that's right for you.
The following specifications will probably meet the basic needs of most users.
Ubuntu 10.04 - with Windows 7 for a dual-boot
Lucid Lynx is the most recent Long Term Support release of Ubuntu, and it's an extremely user-friendly operating system that does very nicely in a laptop setting. Canonical, Ubuntu's maker, recommends the 32bit version for most users. If you choose to dual-boot - not a bad idea if you're new to Linux, are a gamer or rely on key Windows-only apps - you'll most likely get Windows 7 too. Most Linux-focused vendors offer dual-boot installations as well.
A 2.4GHz Intel Core 2 Duo Processor
This is a good, solid, dual-core 64bit CPU that will handle multitasking nicely along with whatever the average user might throw at it. Though you may not need a 64bit processor for Linux, you'll be glad to have it if you're dual-booting with Windows.
4GB of RAM
Again, you may not need this much RAM if you're running a 32bit Linux, but it will stand you in very good stead for most purposes on the Windows side, if you do dual-boot.
Look for wireless hardware from Intel or Atheros, and make sure ahead of time that it's supported with the right Linux driver.
Dedicated video Card
Unless you're very sure that you won't ever be using your laptop for anything involving more than just the most minimal graphics, your best bet is to buy a laptop with a higher-end, dedicated video graphics card. If you'll be dual-booting, that's even more true. The majority of Nvidia and ATI/AMD cards fall into this category, but make sure you confirm before you buy.