I'm currently investigating calendar solutions for a WordPress site I'm building and the phpicalendar / JCM Calendar combination looks really promising.
I have yet to download the code, from the screenshot it doesn't look as though JCM Calendar supports categories. Is there any possibility of allowing users to select a category for an event and perhaps generating a separate .ical file for each one to leverage phpicalendar's features?
I suppose ideally users would be able to create their own categories (possibly integrated into WordPress category management).
Hope this is already something you are working on!