Apple has pulled the iOS app 500px from the App Store. Here is Apple statement:

"The app was removed from the App Store for featuring pornographic images and material, a clear violation of our guidelines. We also received customer complaints about possible child pornography. We’ve asked the developer to put safeguards in place to prevent pornographic images and material in their app."

I'm not sure what "possible child pornography" means. The developers claim they were told their app easily allowed users to search for nude photos. A forth coming app update will apparently rectify the matter. It will be interesting to see how the update will fulfill Apple's guidelines. The developers claim they already pull any pornographic material but do allow for artistic nudes.