I am seeking a GKVoiceChat example for the iPhone SDK in 4.0 or later. It will surely help with my iPhone game.
There are two Game Kit samples I've seen that include GKVoiceChat.
Search for GCPing in the Apple dev forums. (I'm not posting a link because it's not clear the author wants a public link). This sample is clear and basic, but demonstrates a wide range of working Game Kit multiplayer concepts. Search back over 'all' time, and look for the 'GCPing WWDC2010 - Updated for iOS 4 GM Seed' post.
multiplayer Pong client. Found here.
Haven't looked into sample 2 much yet, but it looks pretty good.