This for me.
My wrist pain comes NOT in my mouse hand, but in the left hand from resting on the keyboard doing the WASD work. To explain my specific problem, place your hand and forearm flat on a desk with your hand perfectly inline with your forearm. Now rotate your hand to the left so it creates an angle on the left side of the wrist. Exactly in that spot there is where I feel pain every once in a while.
I've developed it only recently with BF3 while doing some really intense game sessions (i.e. having a death grip on everything while getting to 50 kills in SQDM). I've got one of those cheap $14 wrist curl bars from Wal-Mart and a 5 lb weight plate and I do a couple sets of regular and reverse wrist curls every other day and the pain subsides.
I have a MS Natural keyboard so the keys are split, but for some reason I tend to still naturally rotate my wrist on it.