IOS8 Game Center Sign In Failed
There are some changes in how Game Center will work with IOS8.
First of all, if you want to test Game Center, you need to enable sandbox mode on your device. To do that, open Settings app, scroll down to Game Center and enable Sandbox mode under the Developer settings:
After this you can test Game Center with your game.
If for some reason game center is still not working even with sandbox enabled:
Make sure you created your app correctly and building it with correct bundle id and provision profile.
Make sure that sandbox mode was eventually enabled on the device. Open Game Center App, and you should see sandbox mark like on the screenshot below.