After struggling with adding support for Game Center achievements to LetterMiner, I have decided to stop for now. I realized most of what I wanted to do with achievements I could do with leaderboards. As an example I wanted to have achievements for reaching certain levels, such as reaching Level 5 or Level 10. A leaderboard showing the highest level you reached accomplishes the same thing. Supporting achievements became a roadblock to finishing version 1.0 so I decided to remove the roadblock by dropping achievement support.
I haven’t ruled out adding Game Center achievements in the future. All I can say is Game Center achievements won’t be in version 1.0 of LetterMiner.