A few days ago, I remembered a story by TouchArcade from last year that explained how adding iCloud support to games was no easy task for most independent developers:
It's also obvious to us that iCloud and the implementation of it needs to be easier, and the service itself needs to be more reliable. Almost every studio we talked to had some trepidations or a horror story to share. Browse our message board, and you'll find even more from users receiving the bad end of an iCloud problem.
Today, Google announced Google Play Game Services, a set of APIs to enable cross-platform gaming features like saved game states, leaderboards with Google+ support, achievements, and automatic player matches with real-time multiplayer. There is a native SDK for iOS and Android, REST APIs and various libraries for the web.
WWDC '13 is less than a month away, and it's safe to assume Apple will show new developer tools for iCloud and Game Center. It'll be interesting to see if "Sign In with Google+" buttons will start showing up in iOS games in the next few months.