Ok, sorry, i misread the point...
I'd say "Orb of Fire" will affect the undead to it's full extent, including the secondary effect if it fails the Fort ST. I didn't have time to check, but i think undead are affected normally by the "Dazed" condition.
That being said, i don't know which book is more recent, but the "Libris Mortis: ..." version of the spell specifies it only afects living creatures (in the description). The "Spell Compendium" version, mentions, in the fluff, that vital fluids escape through the skin, and an undead probably lacks that. Also, the secondary effect of this spell is "Stun", a condition which i know undead are immune to.