i used to have that too back when my technique was not as developed. i think its basically a problem of a bad technique. too much tension, too much movement, too much force, etc. when my wrist began to hurt - which was every 6 months or so - irested for a few days and put my wrist in thick immovable bondage and applied some cooling gel on it.
be VERY carefull - i know of alot of guitarrists who had to learn for their final examn at music conservatory and overdid it - some of them have never been able to touch a guitar again - chronical, extreme tendonitis can even hinder you from getting a job for you cant even type on a keyboard! and the usual operations make it even worse.
I only know of one case where a pal of mine was diagnosted "no guitar playing for you evermore" and he couldnt take it, so he spent a year eating vegetables only and drinking like 6 liters of water a day. this helped him beeing able to partially heal the tendinitis and to play again.

[Edited by Azrael on 01-16-2004 at 02:43 AM]