I never had this problem with my T410, T400, T61, T60, or T43, and yes I have the latest UltraNav drivers.
When I'm playing Counter-Strike Source with my T420, the touchpad becomes basically unusable as I move.
You press W to go straight, S to go backwards, A to go left, and D to go right. While you move the direction you face with the touchpad. But now with my new T420, you can use the touchpad the same time your pressing the keys down. You can only do one or the other, not both at the same time. I can still utilize the TrackPoint the same time you're pressing these keys in the game, but not the touchpad.
Anyone have any suggestions/ideas? I've gone through the settings for both the mouse/ultranav in the control panel, as well as the game, but didn't see anything. Maybe I missed something?