Just seeking clarification. Undead are immune to ability damage to physical scores. However ray of enfeeblement causes the target to take a penalty to strength for 1 round per caster level. Is that the same thing as strength damage?
No, not the same thing.
Aren't Undead immune to any affect that allows a Fortitude save though [unless it also affects objects].
Yes, the fact of adding a ST to Ray of Enfeeblement nom makes undeads immune to this spell.
Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless)
I've got a faint memory of RoE having a different penalty to undeads in D&D 3.0, some flat -2 penalty or something like that. Am I going crazy or did it have that?
I think you are confusing RoE with Chill Touch that works differently if the target is undead.