I have found a lot of people having this issue, but the solution never matches my environment.
We are running server 2008R2, Exchange 2010 SP1 and BESX 5.0.3. Problem is that all users do not receive meeting invitations on their Blackberry. It shows up properly in Outlook. If they accept the meeting in Outlook, it will display in the calendar. There are no related alerts on the BES server that I can tell.
This was working at one point. Might have stopped after an MS patch, nobody knows for sure. I have the latest CDO on the BES server. No CDO installed on Exchange.