I'm not aware of any either. I'm not saying it's necessary, This is a multi-Billionaire industry which is constantly evolving and innovating. Look at where they stand now, Almost every Porn site has a iPhone/iPod and iPad version up and running 100% compatible with all iDevices, That's pretty impressive.
Only point I'm trying to make is that Adult entertainment might not be in the App Store today, But I wouldn't be surprised if it makes it's way there in the future. Right now anyone who has Cable or Satellite TV can get full blown access to porn, Is there a need for it? NO is there a demand for it? YES.