Improved Buckler Defense: "When you attack with a weapon in your off hand, you may still apply your buckler´s shield bonus to your AC."
Buckler: "In any case, if you use a weapon in your off hand, you don’t get the buckler’s AC bonus for the rest of the round."
So either:
a/ 'a two-handed weapon' is not 'a weapon in your off hand', in which case the feat does not apply, but the buckler description does not cause you to lose your AC bonus anyway,
or
b/ 'a two-handed weapon' is 'a weapon in your off hand', in which case using a two-handed weapon causes you to lose the buckler bonus, but the feat permits you to retain it.
In either case, someone with a two-handed weapon and IBD retains his shield bonus. In case a/, he retains it even without the feat.
-Hyp.