I’m having a difficult time dealing with meeting invitations from corporate clients. I’m trying a nifty plug-in called You Are Invited, to see if it will help. YAI (You Are Invited) is a plug-in for Apple Mail that translates and transfers Outlook, Google Calendar, Zimbra and Exchange style meeting invitations to iCal. It corrects many incompatibilities between Microsoft Exchange style meeting invites and what iCal can understand. YAI is £3 for Mac OS X 10.4 and up (Universal Binary).
You can download YAI here.