Christof,
we just finished the first version of the Logger application and I asked Timo to help us out adding categorization to that package. It took Timo about 20 minutes to do so. I think the same amount of work would be required for forums, although I think the thing we might be missing is a really nice category browsing widget. When I come to think about it Timo probably developed one and we would just have to use a new template.
