Gees this is the day for Microsoft updates...
Messenger Application Programming Interfaces
The latest versions of the Messenger application programming interfaces (APIs) documentation. Microsoft has developed three sets of APIs that can be used to create custom Messenger applications. For example, you can add images of your contacts or create an add-in component that links to your e-mail system to alert you when new messages arrive.
Download: Messenger.exe (249kb, Messenger API help and associated files)
Download: MessengerAddIn.exe (121kb, Messenger Addin's Compiled help associated files)
Download: MessengerUA.exe (295kb, Messenger User Automation help and associated files)