|Darksol the Painbringer|
I Believe the Intent of Reactive Arcane Shield was to be used even if your flat-footed. As normally you could not use and Immediate action if you're flat-footed(save vs Traps, falls, avalanches, cave-ins and stuff not pertaining to combat).Say you passed your perception check to notice the ambush. That means you know the attacks coming, but are still flat-footed because you rolled low on your initiative to act in the Surprise round. Though this is the only case I could think of that this would apply. If you fail your perception then you can't do anything until you act in regular rounds.
But even if I'm flat-footed, that doesn't mean I can't take actions, much less Immediate Actions.
Here's the Flat-Footed Condition description:
A character who has not yet acted during a combat is flat-footed, unable to react normally to the situation. A flat-footed character loses his Dexterity bonus to AC and Combat Maneuver Defense (CMD) (if any) and cannot make attacks of opportunity, unless he has the Combat Reflexes feat or Uncanny Dodge class ability.
Characters with Uncanny Dodge retain their Dexterity bonus to their AC and can make attacks of opportunity before they have acted in the first round of combat.
So, there's only maybe one part that can be construed, but that is then properly explained by saying you lose your Dexterity Bonus, and can't make AoOs. No sane GM would rule Flat-Footed to mean you can't take Immediate Actions, and it's certainly not something I've seen a thread about from the result of PFS play.