I'd like to mention two simple technical solutions that came to my mind reading the last posts:
1. The cat: namespace is a nice idea. Maybe [[cat:foobar]] could point to the same page as does [[foobar]], so the script could tell apart when you mean "link" and when you mean "category".
2. Another way would be to have no {{}} or [[cat:]] or the like at all, but use the "pages that link here" function I already implemented. Instead of a seperate page, it could be included by a variable on every page that (supposedly) is a category. The advantage would be that we'd only have to add "see also" links or the like on all the pages to have them listed. The disadvantage would be having pages listed that we don't want to have (e.g., "HomePage" would be listed on the "Biology" page, because it links there).
But currently, I don't think the problem is a technological one. IMHO the question is "have category functionality or not?". I'll leave that one to the rest of the community and to the "higher powers";)
Magnus