As far as categorization is concerned I thought you should know that I helped develop quite a feature rich categorization service (any category tree could be scoped to whole site, an object type, or any acs object i.e. a bboard forum) about a year ago at aD in Munich. Even though you may not be able to use the source code directly (it's based on Oracle and aD may not want to release the source code - not my call) you might be able to derive some good ideas from it. You can find the documentation here. I think the requirements document is most useful to read.
Let me know if there are questions!
