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The Android Play Game Services Plugin and the Android Social Networking Plugin now requires Unity 4.2+. We are making this change for all Android plugins that are possible to convert to Unity's newly added library support. This will be a huge boon in avoiding resource conflicts.
If you are using the Android AdMob and the Play Game Services Plugins in the same project it will cause a build issue. For a quick fix, visit this thread in the support forum.
Note that the prime[31] tools are versioned completed separate than all plugins. Each time you import a plugin if you either uncheck the checkbox next to P31RestKit.dll or just run the "Update..." item from the prime[31] menu it will ensure you are always up to date.