Microsoft announced the release by Miyowa of an official Windows Live Messenger (WLM) client for the Android platform. The WLM client is indeed an official messenger app but not developed by Microsoft. Miyowa is apparently a well known company which develops instant messaging apps. It is not the first time I hear about them. In fact they also developed an "official" app for Windows Phone 7.
The application official name is "Messenger by Miyowa". That's how it's listed on both MarketPlace and Android Market. While the app is free, the reviews about it are not great. And I'm still not using it.