Store/Restore Event Categories is a free AppleScript toolset that will allow you to save the categories associated with an Exchange Calendar event so they can be restored in the event they are lost due to the resetting of the local calendar cache.
Installation
Copy the folder to anywhere on your hard disk - the scheduled script can be run from anywhere. However, for easy access, put the scripts into ~/Documents/Microsoft User Data/Entourage Script Menu Items/ (where `~` is you `Home` directory).
Usage
The scripts can then be easily run by selecting them from the Script menu in Entourage (the one that looks like a black scroll to the right of the Help menu).
Store/Restore Event Categories is released under a Creative Commons Attribution, NonCommercial, ShareAlike 2.0 England & Wales License.