I picked up a T500 this evening. The ThinkPads are durable machines, and that is a major consideration for me. I also like the ergonomics of the design. A lot of notebooks these days don't seem to be designed with humans in mind.
I paid extra for the T500 because I wanted a 15.4" display. Unfortunately, I am discovering that the 1280x800 resolution of the T500 just isn't making me happy. My beat up old Dell supported 1680x1050.
But man... that's an awesome price on those T400's and they are very nice mid-range machines.