Great news for Outlook developers--Microsoft has just an online reference and guide to the Outlook 2007 PIAs.  This is the first time there has been a PIA reference for Outlook.  The reference includes a lot of helpful "How To" segments which show you how to do common tasks with the Outlook object model.  You can access it online from the link below, or if Visual Studio is set to use MSDN online help, pressing F1 when over an Outlook property, method, event, or object will bring up the help as well.

http://msdn2.microsoft.com/en-us/library/bb187379.aspx