On August 18, 1999, Microsoft announced plans to publish the protocol for MSN Messenger. It was submitted as an Internet draft to the IETF. It has since expired and been removed from the IETF website, but copies still remain. I have posted an exact copy, and also formatted the text into an XHTML version.
This document is certainly not up to date with the most recent protocol changes, but it is the only official document from Microsoft that is open to the public. It covers protocol version MSNP2.