If you're in the market for a laptop guaranteed to support Linux, the following companies are good places to start. System76 is on top, since that was my choice for my latest purchase. It's a bit early to give a review yet, but things are looking very good.

Oh, only one word of advice: Make sure you go for the Intel option on the Wifi card. The default Realtek card is apparently poor hardware and drivers.

www.system76.com

www.emperorlinux.com

zareason.com

Then there are some list and general reviews:
www.linlap.com
www.linux-on-laptops.com
www.ubuntu.com/certification