The Main 3.5e FAQ has some words on the matter. Sadly, the words are a bit ambiguous.
Now that you've read the PH rules, read the FAQ, and talk with your DM to decide if
(a) armor spikes ALWAYS inflict extra damage after a successful grapple check (which is what the PH says),
(b) wearing armor spikes while grappling allows you to choose to inflict either piercing damage OR unarmed strike damage but not both after a successful grapple check (which is one way of reading what the FAQ says), or
(c) even though you always inflict unarmed strike damage, wearing armor spikes while grappling allows you to choose whether or not you inflict to inflict extra piercing damage after a successful grapple check (which is another way of reading what the FAQ says).
There are situations in which you don't want to inflict lethal damage on the creature you're grappling. The FAQ entry might be construed as adding a new option (c) that lets you choose not to do so.
USD = Unarmed Strike Damage
EPD = Extra Piercing Damage
(a) = USD + EPD
(b) = USD or EPD but not both
(c) = USD + optional EPD
