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