I have a new server setup with new workstations running Outlook 2007. On several of my users, when a Calendar item is created in Outlook, it cannot be view in Outlook Web Access. Anything created in Outlook Web Access works fine. All users are setup with Exchange Cached Mode turned on.
Let's say I take UserA and log her onto another PC, create an Exchange account also in Cached mode, and then open Outlook. The Calendar items that were created on UserA's other PC using Cached mode *disappear* from the Calendar altogether. Of course, any item created using OWA remains.
Turn Cached Mode off - items remain.