The question arises because someone referenced the wording on the feat's prerequisites, they said the Druid would need a natural attack all the time to qualify:
Improved Natural Attack:
Attacks made by one of this creature’s natural attacks leave vicious wounds.
Prerequisite: Natural weapon, base attack bonus +4.
If the Druid does not have a natural attack in his regular form, does this prohibit taking the feat? I perceived it as similar to taking ranks in the Fly skill–you just have to have access to a form that can do it–but I could be wrong.
Best Answer
From the Paizo SRD on Feat Prerequisites:
It appears that you can take the feat, but can't use it while not having a natural weapon. You regain the benefits when you regain the prerequisites.