I have an x61s, and highly recommend it. Lightweight (<3lbs), durable, great battery life, performs well (Ubuntu 7.10). If you are willing to wait a few months, a mac ultraportable is in the works and likely to be released:
http://www.macrumors.com/2007/12/04/apple-orders-13-3-led-ba...
The two constraints you have that will conflict the most are lightweight and large screen. If you are using this as a primary desktop replacement, I'd go with a 20"+ LCD for desk usage.
as the other poster said, you are going to have a hard time finding such a large screen in such a small package.
just get a nice lcd monitor, keyboard, and mouse. when you're at your desk you have a desktop, and when you're out and about you have a small little window into the internet.
imo, though, the thinkpad x series is the best out there. amazing how well engineered it is.
The two constraints you have that will conflict the most are lightweight and large screen. If you are using this as a primary desktop replacement, I'd go with a 20"+ LCD for desk usage.