Where does it say it's an ability? I see "All fear attacks are mind-affecting fear effects" here, but that's not to say it's an ability. Give me a page number or link, please. 3.0 intelligent undead are immune to intimidate, I agree. However, I'm inclined to believe they aren't in 3.5.

And I respectfully disagree with fear is fear, no matter how it is classified. To say that is to say fire produced by any magical spell is the same as real flame.