Just received this notice at work:
Outlook Calendar Issues with iOS Devices (iPad and iPhone)
There is currently an issue with Exchange calendars on devices running iOS 6 (iPhones and iPads). The issue manifests itself when a meeting recipient declines an invitation, which then cancels that meeting for everyone. Apple and Microsoft are aware of the issue.
For now, until the issue is corrected, we recommend users with iOS devices manage their calendars from the Outlook client on their computers.