I used Cocos2d and Ganbaru Game's Game Center helper to integrate Game Center into Dead End, an iOS game of mine.
However, upon converting the game to Universal, I found two problems.
1. GameCenter wouldn't pop up on iPad.
To resolve this, you have to change the modalPresentationStyle for your leaderboard. The default does not work on iPad.
So this is my showLeaderboard code, with an added line for iPad/iPhone integration.
2. When I was dismissing the leaderboard on iPad, touch controls stopped working.
On iPhone/iPod, the dismissal code looked like: