It was answered in the FAQ early on, I believe. But the most authoritative source I can find in under a minute is Heroes of the Forgotten Kingdom:
Which is pretty straightforward and clearcut on the matter.
This is actually part of a larger, more general rule that you only add extra damage and damage boosts if there is actually some base damage to add it to. So your gnome wizard doesn't add damage onto Illusory Obstacles (which does no damage), even though he has a feat that adds bonus damage onto any arcane illusion power he casts.
Thanks for quoting the HotFK. I don't have that book so wouldn't have been able to look it up. Hopefully its in Rules Compendium should it come up and I think it needs to be enforced (would be more likely I suppose if there were a Pacifist Cleric). Anyhoo, I knew the issue about needing a damage roll (generally) in order to add extra damage, just had never seen it connected to the crit.