TrizzlWizzl said:Right, but to start a grapple you gotta make a melee touch attack which provokes an AoO (right?). So the original poster is asking whether or not the shield bonus applies during an attack action which would provoke an AoO.
Assuming I have the question right, I'd rule that if someone readied an action to attack the caster as he's attacking, then no... the shield bonus doesn't apply (it 'moves out of the way'). So if the shielded caster makes an attack that provokes an AoO, his shield bonus doesn't apply.
There's nothing wrong with ruling it that way (it's your campaign, after all), but I don't think that's the intent of the spell as written. You can do a hell of a lot more with a readied action than just starting a grapple. In fact, I'd say that grappling is one of the less likely options on a readied attack -- better to hit the guy with an axe, or an arrow. If you're going to grapple, you might as well just do it, instead of using a ready action.
Furthermore, a readied attack against a casting attempt already forces a Concentration check to keep the spell up. This alone is a very good reason to ready attacks against spellcasters. Doing it the way you suggest would remove a lot of the point of the shield spell.