Version:           4.3.0 (using 4.3.00 (KDE 4.3.0), Gentoo)
Compiler:          x86_64-pc-linux-gnu-gcc
OS:                Linux (x86_64) release 2.6.30-gentoo-r2

How to reproduce:
→ Create a new event
→ Open the categories selection dialog
→ Assign a category
→ Create a 2nd event
→ Open the categories selection dialog
→ Apply the same category used in the first event
→ Rename this category using the "Edit Categories" button
→ Apply the changes
→ Open the 1st event again
→ No category is selected

It seems that the old event still points to the old category name, while this category isn't found anymore in the global categories list.
So this event points to a non-existant category.

I think the whole category system could need a rework.
Currently just the plain category name is stored for each event.
This doesn't just cause the trouble described above, but makes it also hard exchanging events between multilingual environments.

I think each category should just have an internal UUID to which the assignment is made.
So it would be possible to
- rename categories without loosing the assignment in events
- translate events for better collaboration in multilingual environments

The same problem applies to the Addressbook - I don't know whether KOrganizer and KAddressbook share the implementation, but if this is re-implemented, the same could be extremely useful for KAddressbook too as I'm experiencing exactly the same problems there too.