MAPI documentation has been moved from Exchange tree to Outlook tree with Outlook 2007.
The documentation has been updated and the links are compiled below.

Welcome to the Outlook 2007 MAPI Reference
http://blogs.msdn.com/stephen_griffin/archive/2008/12/02/welcome-to-the-outlook-2007-mapi-reference.aspx

FYI: The MAPI docs have moved...
http://blogs.msdn.com/mstehle/archive/2008/12/04/fyi-the-mapi-docs-have-moved.aspx

MAPI documentation moved to Office
http://blogs.msdn.com/exchangedev/archive/2008/12/04/mapi-documentation-moved-to-office.aspx

MAPI Properties and the New Outlook 2007 MAPI Reference
http://blogs.msdn.com/officedevdocs/archive/2008/12/03/mapi-properties-and-the-new-outlook-2007-mapi-reference.aspx


Welcome to the Outlook 2007 MAPI Reference
http://msdn.microsoft.com/en-us/library/cc765775.aspx

Outlook 2007 SDK: MAPI Reference
http://www.microsoft.com/downloads/details.aspx?FamilyID=4b1b8b22-3c74-4479-bd0d-7b0ee29e8d59&displaylang=en

MFCMAPI
http://mfcmapi.codeplex.com/

Microsoft Exchange Server MAPI Client and Collaboration Data Objects 1.2.1
http://www.microsoft.com/downloads/details.aspx?familyid=E17E7F31-079A-43A9-BFF2-0A110307611E&displaylang=en