I saw an interesting use of MSN Messenger the other day , where it was used to create an IM bot. The peaked my curiosity, so I downloaded the DotMSN library to experiment with. Considering it's based on a deconstruction of the undocumented API, it works really well.
However, as it's unofficial, it's risky using it for a business application, as Microsoft could make breaking changes, so I was glad to see that the msdn forums had added a new forum for the Messenger API. Earlier today, the first post was added announcing that the official API will be released on Monday 12th September.