By defining categories and tagging contacts according to their category, I was hoping to be able to produce a list of contacts filtered by category. Is there a way to do that? Ideally, I would be able to export a subset of contacts in csv format for use in generating a printed address book for my Daytimer.

I'm not sure KAddressbook offers this kind of functionality. The backend is likely capable of doing what you want, just there's no UI for it.

It is easy to filter by Category, just type the category in the search field, but if/what you can do with the output I have no idea

That is very helpful, and with Export>Selected I can create the .csv file. The only problem is that the custom fields aren't exported. I can get around that, but it would be very nice if the Export function included the custom fields.

Happy to be of assistance! Unfortunately I have no idea on the fields issue, hope someone else can pitch in or if you should file a bug/wish perhaps?