actually, that recent thread wasn't about using a powerbook for CAD
I have a friend who uses an older G4 15" powerbook for CAD. He uses it on a daily basis in a business setting for both 2D and 3D work, although the 3D models aren't especially complex. His powerbook is slower than the current models you would be buying, but he says it works great for his needs.
If you are thinking of doing large, highly complex 3D models that you want to render to the highest quality, then maybe a dual G5 PM would be a better choice, especially if you are thinking of using it for productition work for a business.
I have a 12" G3 iBook that I use for CAD occassionally.....I think the screen is too small for everyday use, but then I'm used to a 19" screen with a G4 PM for my usual CAD work.