I'm trying to get my head around all the changes in the Outlook 2007 object model. Some good resources are found on MSDN:

New Objects, Collections, and Enumerations
New Members and Constants
Object Model Changes Since Microsoft Office 2003
Developer Issues When Upgrading Solutions to Outlook 2007

Also, this may be helpful to someone out there.  I created two idl files, one with the Outlook 2003 object model and one with the Outlook 2007 object model.  I sorted the idl files so all the enums are listed first in alphabetical order, then all the interfaces in alphabetical order, then all the coclasses in alphabetical order--this gives you a nice view when you use windiff or some other differencing tool to compare the two object models.  Note that these idl files have some objects removed--specifically, I removed most of the objects having to do with built in outlook controls you can use in a form region because my focus is using managed controls in form regions.

Outlook 2003 sorted IDL file

Outlook 2007 sorted IDL file