I'm adjacent to an opponent that threatens me. I use the feat Spellsong to conceal that I'm casting a spell. My Perform check result bests my opponent's Perception or Sense Motive skill check. Do I still provoke an attack of opportunity from that opponent?
You still provoke, but not everybody will be able to attack
Enemies do not get an Attack of Opportunity (obligatory reference) because they notice a spell being cast. They get an Attack of Opportunity because they notice you aren't paying enough attention to properly defend yourself.
You can combine your bardic performance and your spellcasting in two ways. First, you can conceal the activity of casting a bard spell by masking it in a performance. As a swift action, you may combine your casting time of a spell with a Perform check. Observers must make a Perception or Sense Motive check opposed by your Perform check to realize you are also casting a spell.
A few things to notice here:
- Bardic Performance is a supernatural ability, and those do not provoke;
- Enemies are allowed an opposed skill check to resist the effect;
- You are still casting a spell, regardless of the effects of the feat;
With that said, enemies who pass their check will notice your ruse and attack you normally, but those who fail their checks are denied their attack of opportunity. It simply doesn't look like you guard is down when you are making your performance-spellcast, and as such, enemies who are not succesful on their check will not see an opportunity to attack you.
Similarly, when you are casting a silent or still spell (or a spell-like ability) you still provoke attacks of opportunity because you're zoning out on the battlefield and letting your guard down.
There is another feat that has a similar effect, Conceal Spell (from Ultimate Intrigue), though the interactions with Attacks of Opportunity are much clearer in there, it was also published 5 years later, so they had time to learn a few mistakes when presenting game-changing mechanics like that.
Since you are concealing the spell’s manifestation through other actions, others observing you realize you’re doing something, even if they don’t realize you’re casting a spell. If there is a verbal component, they still hear your loud, clear voice but don’t notice the spell woven within.
If an opponent fails its check, your casting also does not provoke attacks of opportunity, and an opponent that fails its check can’t use readied actions that depend on realizing that you’re casting a spell or using a spell-like ability, or readied actions such as counterspelling that require identifying the spell you’re casting. Spells such as fireball that create an additional obvious effect (aside from the manifestation of casting that all spells and spell-like abilities share) still create that effect, though it might not be obvious who cast the spell unless it emanates from you.
Bardic Performance does not, by itself, provoke attacks of opportunity. So, I would expect that, if the opponent failed to detect spellcasting thanks to Spellsong, then no, they would not get an attack of opportunity.
This would be similar to, say, a bard successfully making an opposed Stealth check to hide behnd some cover. If the check succeeds, the opponent isn't aware of where the bard is.
You still provoke.
Think of it like this: the opponent does not get to attack you because you are casting a spell, they get to attack you because you are letting your guard down (and you are letting your guard down because you are casting a spell).
The Spellsong feat hides that you are casting a spell, but it doesn't prevent you from dropping your guard. It would have to explicitly mention that it also prevents attacks of opportunity to do that.
So you still provoke, the enemy just isn't sure why.