Microsoft released preliminary protocol documentation for Exchange 2007 (as well as Microsoft Office 2007 and SharePoint Server) as part of Microsoft's Interoperability Principles. The Exchange protocols are now publicly available on the Open Protocol Specifications section of MSDN in preliminary form for developers to review and provide feedback.  You can read more about the announcement in this press release.

Here is the press release:
    Microsoft Publicly Posts Additional Protocol Documentation
    http://www.microsoft.com/presspass/press/2008/apr08/04-08ProtocolPR.mspx

Here are links to the documents:

    Exchange Server Protocols
    http://msdn.microsoft.com/en-us/library/cc307725(EXCHG.80).aspx

    Exchange Server Protocol Documents
    http://msdn.microsoft.com/en-us/library/cc425499(EXCHG.80).aspx

I cannot answer any questions on these documents. Support for these are done only by forum and are preliminary. 

    Using the Exchange Server Protocols
    http://forums.microsoft.com/MSDN/ShowForum.aspx?ForumID=2084&SiteID=1

This is a great leap forward in documentation and I'm thrilled.