Does anyone know if the feat 'Skill Focus' would apply to all forms of the 'perform' or skill, or just to a particular sub-skill?

I'm sure this query is answered in one of the faqs but since the dastardly advent of 4th ed. it's hard to find them.

I don't know for sure, but as a DM, I would have it apply to only one form of the skill. You can sing very well, but that doesn't mean you can dance very well either.
This would be my ruling, unless the bonus is an insight bonus, and then I would let it apply to all forms, since it's more about a bonus on you intelligently manipulating the crowd by entertainment.

That's exactly how I would expect it to work. But it would be good to know if there's an official ruling anyone has come across on this.

I believe that since 3.5 all subskills of perform must be take separately, that skill focus treats them each as a separate instance of the skill and would thus require multiple feats for multiple bonuses to apply.

I just KNEW I should have save the answer I got from WoC about the very same question!

According to what they told me, the Perform skill entails choosing a sub-division of performing (drama, comedy, music, oratory, etc.), just like a Craft skill subdivision (Trapmaking, armorsmith, metalworker, etc). Therefore, the Skill Focus feat only affects a particular sub-division of performing, just like it only affects one craft ability and not all.

Ah that's good to know. Thanks for that!