It’s easy to combine post types and taxonomies, but it’s difficult to come up with a UI that makes sense for both. Search the WordPress codex on how to combine post types and taxonomies. There are a few good examples out there.
Ok thanks. Does the bbpress plugin use custom taxonomies for topic tags? Because if it just used good old tags (read old school taxonomy), wouldn’t my problem be solved?
Maybe a feature request/idea for future releases of bbpress.
It’s only a problem if you want it that way. 
bbPress is going to keep its core taxonomy separate from WordPress posts. If you want to merge them together, it’s completely possible to do on your own.
Would be interested to know what solutions you come up with for this tntc1978.
Since my coding skills are modest I went ahead and asked Justin Tadlock (you need to be logged in to see his answer (best bucks you’ll ever spend)), to whom I always trust a reflective answer.
But he’s answer only gave me little hope as well. So for now I’ve shelved it.
I wonder if anything has changed on this since last year.
I’ve got a WordPress Custom Taxonomy that i’d like the forum, topics and replys to be tagged with – has anyone ever got this working?
I’m in the same camp as @Nova; I’ve got custom taxonomies that I want to be included with forum posts.
In the meantime I’m just creating my own post type “discussion”, i.e. faking a forum with wordpress posts.
(the trick there is to find/create a view where posts are sorted by “last comment”)
Did you figure anything out? (Nova or anyone else…)
I was able to get a custom taxonomy associated with topics only partially working with bbPress 2.6a and WP 3.8, but the performance of queries was terrible compared to using postmeta. A query using a custom taxonomy that took 3.8 seconds took only 0.01 seconds with the same information stored in postmeta. So performance needs to be taken into account.
