The recent Rules of the Game on grappling said that you could use your shield while grappling, but at a penalty to your attacking grapple checks. It doesn't unfortunately go on to say whether the AC bonus still applies. Arguable since it is covering your body, it should still apply, but since you're not wielding it, it may not. My intuition says that it's covering you so the AC bonus should apply. You do after all suffer the penalty to your grapple checks for that benefit.
Pinotage