+1 For the Dell Precision, although it's very much a 'portable workstation' rather than a truly mobile laptop. Think of it as something you will use with a docking station in place of a full tower.

Also, you'll probably want to know about this fix: http://askubuntu.com/questions/50491/detect-touchpad-as-touc...

Dell Precision machines are other worldly (hope Dell doesn't go under, got an M4700 for development and a couple of R610s in colo, yelp).

Linux support for PC laptops should be pretty decent across the board these days, IMO. Will require some tweaking for things like trackball and trackpad, but otherwise basic system functionality should be there out of the box.