meeting events from Mac iCal users not getting added to calendar

I have contacts that use the Mac OS iCal program. They send meeting
invitations from iCal via e-mail (those invitations are in .ics format, which
supposedly Outlook can use). However, whenever the event is double-clicked on
it does not allow any options for adding it to the Calendar. If the .ics file
is sent as a zip compressed attachment and then opened it works fine, but
..ics events sent from iCal directly are not getting properly handled by the
mail portion of Outlook.

I am running Outlook 2007 (12.0.6514.5000) SP2 MSO (12.0.6425.1000) (but
this was also a problem when I was running Outlook 2003.

I know that there is nothing wrong with the .ics files from iCal as they
work just fine when protected in a .zip archive, but it's not very convenient
for either the sender or myself to have to deal with another level of
abstraction in order to share meeting event invitations. Is there a fix for
this. I've checked the knowledgebase and this issue has been addressed a few
times, but non of the fixes work with my version of Outlook.

Thanks for you assistance.