...We'll be publishing a KB article to establish this support policy update on CDO 1.21.  I'll update this post with KB number, in the meantime, here is the content of the article...

Calling the Respond method to accept or decline a meeting request using CDO 1.21 is only supported through the MeetingItem interface (http://msdn2.microsoft.com/en-us/library/ms527208.aspx).  It is not supported to call Respond to a meeting through the AppointmentItem interface (http://msdn2.microsoft.com/en-us/library/ms526385.aspx) or by calling Respond on an AppointmentItem retrieved from the GetAssociatedAppointment call of the meeting request.

 

...This has to do with the logic implemented in CDO 1.21 to accomodate changes in the calendaring logic of Exchange and Outlook described in the following article, http://msdn2.microsoft.com/en-us/library/aa363947.aspx.