Did you find a fix for this since you posted it?
Here is a description of my problem - which I am interpreting as the same as yours:
I have had a problem with my work team members not being able to accept my iCal invitations in their outlook client tools. I really need a solution to this problem. In general I have found the Mac to be the most 'interoperable' tool that is possible. Most of the government clients and contractors I work with have thought that I was swimming upstream by basing my business on a MBP - and have been amazed I have done so successfully thus far. Now I am worried that the interoperability I have seen in the other Apple software products (i.e., Keynote, Numbers, Pages) is not there for the 'calendar' environment I have to work within (e.g., MS Outlook/Exchange, Google, etc).
Please - if anyone reads this post, reply to it with your solutions or observations on the problem.
I have noticed that there seems to be a lot of posts in this community without any replies. Makes me wonder.
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.
Oh, the duties for sending out the calendar invites have shifted to a person with a MS machine. I can accept their invite if sent to my gmail account - which I suck into the mail client. Havenot tested if I accept the same invite on my iPhone if it will post it to the calendar (and therefore sych my MBP's iCAL via iCloud). Will probably do that next week.
Interestingly, I also suck another mail system, that is MS exchange based, into the MBP mail client, but can't get that to post invites to iCAL at all. Have not tried to look into that wrinkle yet either.
Thanks for the help Tom. It looks like I'll have to go back to using busy sync and google calendars....Once I set it up I do everything through ical, then it pushes to google calendar, which then I'll sync to my phone. That's what I was doing before with an Android phone. I just thought now that I had an iphone iCloud would make all of that better. Oh well. The only trouble I had with google calendar and busy sync was that invitees responses wouldn't make it to my calendar -- I would get the email saying if they accepted, declined or were tentative, but I would have to track that way.