Left: T40, Middle: T60, Right T500. We have a T61 in the lab also but it didn't show up for the family photo. The T60 is mine. Compared to the T40, the only things I miss are the red and blue stripes on the mouse-keys 🙂 On the T500 there are lots of things that would be nice: Built-in webcam, built-in 3G, FireWire, and a HDMI-port. But there's also a lot of keyboard flex on the T500 compared to the older models.
The T400 would still be a sensible choice if I decide to upgrade (althougn I'm not thrilled by going from a 1400x1050 screen to 1440x900). If I win on the lottery any time soon I'll go with an X301.