Marshall said:
Well theres another problem. Just why didnt that shield spell block your Sunder attempt? Isnt that what the spell does? What about my Ring of Deflection +5?
For that matter it brings up another instance when Sunder is useful : When the opponents AC is high. Take away his weapon and you take away his ability to do damage to you and you open him up to other touch AC attacks. That happens often and its usually easier to take out the guys weapon than just hit him.
Well, in this case, the shield spell might arguably count towards the bow's AC since attacking a bow is technically attacking a held object according to the Sage (and I agree). That section of the PH doesn't specify that cover bonusses apply to the AC of a held item but common sense suggests that some cover might apply to the object. The rules do specify that deflection bonusses apply so the ring of deflection +5 would help as well.
WRT melee weapons, shield has no effect upon sunder since they are attacking your weapon and not you. The shield spell produces a magical disk of force that protects your body but moves out of the way of your weapons. As a side effect of that (and the fact that the game rules specify an opposed attack roll--in which AC plays no part), the Shield spell does nothing to protect against Sunder. (And should not). It would be a fair house rule to apply any nonmagical cover bonusses to AC to the defender's attack roll in a sunder attempt though. In fact, I'd probably do that in the case of nonmobile cover.
As for Sunder being useful when the opponent's AC is high, the opponent's AC has to be higher than 10+[opponent's attack bonus] in order for this to be true. (Otherwise, your odds are just as good for hitting the opponent as for breaking their weapon). In the case of a defensively maximized fighter using expertise, this may be true. Similarly, it will often be true in the case of a defensively maximized fighter/wizard.
Another entry into the calculus is the opponent needs to be using a non-metal hafted, non adamantium weapon. Even with improved Sunder, there'd be no point in trying to hack through an adamantium heavy mace (or in my interpretation, an adamantium light flail, heavy flail, or spiked chain).