We received a tip and a screenshot (seen above) from a developer who pointed out the new "Explicit" category in the tool used to submit apps into the store.
We will update this story as it develops.
Update: The new category is corroborated by Cult of Mac and MacRumors as well.
Update #2: Cult of Mac reports the category appears to have gone missing, although we've heard that when it was available earlier choosing the category wouldn't stick -- so perhaps we're seeing some iPhone OS 4 action here?
Update #3: We have independently confirmed that the Explicit category isn't showing up in iTunes Connect any more, as has The Register.