I've tried the IBM ultraportable (no CD/DVD) and the Dell, and wound up liking my Sony Vaio TX Series the best. It's extremely light (2.84 lbs), 11" WXGA screen at 1366x768, and a keyboard that is only slightly smaller than normal and very usable. It's one of the few ultraportables with a builtin DVD, which is important for me because I hate carrying around an external drive. I get about 3.5 hours on the battery, and since I have a spare I have plenty of power when I'm on the plane. I haven't seen anything else (include the new Mac or the new IBM) that would make me give it up. Runs Gentoo beautifully.
I basically use it all day, every day, not just when I travel, that's how much I like it. It's so light that I have to double-check to make sure I have it with me.