Tumblr is banning all explicit content as of December 17th. All posts with explicit content and nudity will be banned from the site. Tumblr has also updated its community guidelines to reflect this. All current posts on Tumblr containing pornographic material will be converted to private mode. The only nude material that will be allowed on the site is of classical statues and political protests.
Tumblr has announced that this is to stop porn-related communities from using their platform. This site overhaul does seem to be happening very quickly after Apple removed the app from the app store due to child-porn related material found on the site. This definitely seems like an attempt to get back into the good graces of Apple and hopefully get their app back in the store, especially with them updating their community guidelines to help protect them if this material does get uploaded.
This removal from the Apple app store has had a greatly negative affect on Tumblr, which seems to be why the company is scrambling to make major changes to their site. Despite the controversy, the app is still available on the Google Android store.