The point, dvvega, is not whether undead are objects; they are not.
However, creatures without a constitution score (like all undead) are immune to effects that require a Fortitude save unless they also affect objects. Hence, Mad_Gulag is correct even though unsure about it: destruction does not work on undead.
Either that's only in the case where they HAVE to make fort saves (I think this was a Suggestion somewhere, and not a rule), or that is only in situations like an Undead raging ability, or an Undead Psionic Lich who used Con in life.
Actually they only do that with the Concentration skill.
Check a sample of Undead Fort Saves. They are all ridiculously low because of the lack of Con. The rare Fort Spell that DOES effect Undead defintely puts a whuppin' on them.