Why doesn't a Google search for "list of laptops which debian runs on" yeild a result which includes this page: Installing Debian On...?
This blog post exists solely to try and rectify that situation.
This article called How to Buy a Laptop for Debian Linux also seems quite informative and is generally the set of steps that I follow when buying a new laptop. One thing that sometimes constrains me is finding a laptop that is aesthetically pleasing and well designed and is also able to run Debian GNU/Linux out of the box. The most important bits of hardware to be sure of running under Debian are usually the video card and the wifi card.