I believe I have learned that the problem lies in that MS Outlook does not comply with the calendar standard, for which Apple is firm on. I wonder if this means that you will find the same problem with Google Calendar, since I think they too are based on the same standard.
I have also learned that this has been a recuring problem for a long time. Looks like that eventually MS fixes by making sure Outlook can handle the iCal meeting request - but I am not sure if that solution was also dependent on IT departments ensuring they are current with the updates and such (probably).
I suppose that means MS has not caught up to the Lion release (which probably was based in part on an update to the cal standard), or atleast the people we are dealing with are supported by IT departements who are not up to date.
I also learned that in the past there was a way to modify mail.script, but I have not been successful in doing that. My failure appears to be related to a permissions issue - I thought I had done what was needed to give myself the permissions to change the file, but the changed permissions are not 'taking'.
There is also a 'note' in that script file that made it sound like it was no longer used (so why is it there?) - that iCAL requires the file to to be manually recompiled as part of the iCAL application complie. I'm just learning about that so not sure.
Ran out of time to keep digging, but plan on picking up the shovel again later.
So I have no solutions for you yet, but thought I'd share what I had learned so far.
Regards
t