I had really bad wrist pain last spring after starting to do more mountain biking with straight bars. They felt fine on my road bars, but the tendonitis or arthritis (or maybe a combination of both) was very uncomfortable for several months. I changed the grips on my bars to Ergon WP1 grips and it really made a difference. They have a wider, ergonomically-shaped platform that gives my hand more support and puts less stress on the outside (ulnar side) of my wrist.

http://www.ergon-bike.com/ergon_wp1.php?la=en