We have multiple users with iPhones running the most recent 3.x OS version attached to an Exchange 2003 server (SP2) through ActiveSync. If you send one of these users a meeting request, and they accept the meeting request in Outlook on their PC, it does not update the meeting request on the iPhone to show that it has been accepted. On the iPhone it still shows the options to accept/deny the meeting. In fact, if you reject the meeting request from the iPhone, it overrides outlook and deletes the event off the calendar. The opposite does not hold true, however. It does not seem to matter whether the event is sent to the user as part of a distribution group or individually.
I suspect this is a known issue with iPhone-Exchange but I cannot find any documentation specific to this scenario. This leads me to wonder if there is anything on the Exchange server that I can check/configure to remedy this issue.