I still can't believe that MobileMe doesn't offer collaborative calendars....
They just lost $500+ from our workgroup.
Hope this helps someone else
Keep in mind that MobileMe was never meant to be a collaborative tool. It was meant to be a personal service, syncing one account to many devices.
While it unfortunately didn't serve your needs, it's really not a shortcoming with the service, since the service was never meant to be used in that manner.