I run Ubuntu on a MacBook, but as long as the laptop you buy has Intel graphics and wireless, you'll usually be fine. My last laptop was one of Dell's 12" widescreen models, and it worked fine for me.
have you run into any problems with the built-in wireless? i know the older powerpc based models did not have a linux driver available for the wireless, but i'm not familiar with the intels.
The wireless works fine for me. I think I've come across a post or two on forums about the newest MacBooks having issues with the wireless that force people to use ndiswrapper, but mine works fine. I wouldn't suggest getting a Mac if you know you're going to be running Linux all the time. I got mine because I got a pretty nice discount. I never touch the OS X partition.