A character who has not yet acted during a combat is flat-footed, not yet reacting normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.
This is part of the Revised (v.3.5) System Reference Document. It is covered by the Open Game License v1.0a, rather than the Creative Commons Attribution Sharealike License. To distinguish it, these items will have this notice. If you see any page that contains SRD material and does not show this license statement, please contact an admin so that this license statement can be added. It is our intent to work within this license in good faith.