NeuLion (who make the app for iOS and android confirmed in a tweet to me that an app for WP was 'in the queue' when I enquired last month. Fingers crossed it'll be here in time for the upcoming season.
10 bucks says it was built for 8.1 and therefore Verizon won't allow it to be released until all their devices have 8.1
While they continue to refuse to release 8.1 on any of their devices
I called them and they said there is no current plans for any Windows Phone game center app.
I love the Windows phone, but when my 2 year contract is up, I am switching to an Android phone, because of the lack of needed apps in Windows 8.1 format.