This is a good move, but only if it's the first of several steps. Adding an EXPLICIT category by itself does nothing to stop the unscrupulous developers out there from adding their nudie-pic app to the games, entertainment, lifestyle, or even business categories. Even if Apple forces it into the explicit category automatically before release, the developer can go back in and change categories at any time without submitting it again. Tired of lack of visibility in the porno section of the App Store (which will quickly rival games and entertainment in size)? The dev moves it elsewhere so he can rank in the Top 100 of a smaller category.
Developers are doing this spammy crap every day. Take a look at the Top 100 Role Playing Games in the app store. It's a damn mess. Probably half the stuff listed there does not belong in RPGs. This hurts the growth of real RPGs even further. Why create a new RPG if it'll be spammed out of the top 100 by crap that does not even belong there?
Basically, Apple needs to take control of categorizing ALL apps and games away from the developer. It's the only way to give categories any real meaning going forward. I've brought this up with Apple directly, and I think others have too. It'll be interesting to see if they do it.