Please READ the posts above from Cookie and ShawnZ!
It is a function in some addons to MSN Messenger (such as the above quoted MessengerDiscoveryX) which changes your contacts name, font and font colour to match yours!
Thus when you get the message, it LOOKS like you sent it, hence the name "Imitate".
Thus if half your contacts are doing it to you, then it is more than likely they also have such a program installed.