Ahrimon
Bourbon and Dice
SRD said:Impromptu Sneak Attack: Beginning at 3rd level, once per day an arcane trickster can declare one melee or ranged attack he makes to be a sneak attack (the target can be no more than 30 feet distant if the impromptu sneak attack is a ranged attack). The target of an impromptu sneak attack loses any Dexterity bonus to AC, but only against that attack. The power can be used against any target, but creatures that are not subject to critical hits take no extra damage (though they still lose any Dexterity bonus to AC against the attack).
Does this apply to ranged touch attacks from casting a spell? I would say yes because it doesn't say that it has to be part of an action. But I'm not sure because it mentions melle and ranged attacks, but doesn't specifically mention thier touch versions.