This has come up several times and thus far our DM has ruled against it. The question is: can a player take an immediate action against another player's or creature's immediate action, so long as conditions are met, like not being on their own turn?
For example, if one of the other players moved in to flank a red dragon and the dragon decided to tail whip [Tail Strike (immediate reaction, when an enemy moves to a position where it flanks the red dragon, at-will)], could I then, as a Bard, use Arrow of Warning?
Trigger: An enemy within range makes an attack roll against an ally
Target: The triggering enemy
Hit: 3[W] + Charisma modifier damage. The ally who was attacked by the triggering enemy can make an at-will attack against that enemy as a free action. The ally gains a bonus to this attack roll equal to your Wisdom modifier (minimum 1).
Our DM has said many times that you cannot take an immediate action against another immediate action. In the end, as a DM, it's his call anyway, but straight from the rules none of us can find a specific answer against it, so we would assume it is allowed.