Apple released a software update on November 1 for iPhone/iPads running IOS version 6. This is a recommended and critical update for anyone using ActiveSync for Calendar scheduling. There was a known issue in the initial release that caused calendar entries to disappear.
The IOS 6.0.1 update is available on iTunes as well as wirelessly.
For more details on IOS 6.0.1 see Apple’s Knowledge Base: http://support.apple.com/kb/DL1606