Hi Xorjo,
I seem to have the same problem.
When I SMS with my wife (iMessage) she get's weird responses to our conversation from my phone, which I have never written, and that we can only see on her phone in our conversation.
And apparently who ever is listening in, they can read our conversation, since they are answers to our conversation - but I never wrote some of the texts she gets in the middle of our conversation.
What's going on and is this a bug in iMessage or what?
We are both on the latest iOS 5.1 and i have an iPhone 4 and hers is a 4S.