This isn’t really a “fix” as it was something that I learned that was causing me a headache. My wife just got a Samsung DROID Charge and was sending me “meeting requests”. These appointments would be automatically added to my calendar without me accepting them. I finally realized that I had both Mail.app and iCal running on my iMac. I looked through the settings in iCal and found a setting called “Automatically retrieve CalDAV invitations from Mail”. After unchecking this, my problem is fixed. I guess, that setting needs to be unchecked so I can see invites before they actually happen.