Taxonomy Defaults breaks forum module's ability to pre-select terms

Pedro -- I'm having a little trouble understanding your English. It would help if you wrote down the specific steps you took, from the very beginning, and indicate exactly where the results differed from your expectations, so that I can reproduce the problem and address it.

Here's a good example of helpful steps: http://drupal.org/node/559828#comment-1975342

Had same issue, giving particularity of forum nodes, taxonomy default should have nothing to do with this.
Default term (eg. forum id) is assigned by forum module in forum.module @217:

    case 'prepare':
      if (empty($node->nid)) {
        // New topic
        $node->taxonomy[arg(3)]->vid = $vid;
        $node->taxonomy[arg(3)]->tid = arg(3);
      }
      break;

Subsequent elaboration of taxonomy defaults ovverrides this, assigning no tid to new forum post (even if one term is set in config), which is then left alone, eg. not listed in forum.
I tried a brute solution excluding the treatment of new nodes of "forum" type (patch attached).
Also configuration options should be removed for forums, since they are no more effective
I know it's not elegant, but at least it works for me.

What would the ideal behavior be here? What's the user scenario?

As far as I can tell, the only reason to do this would be if you have only one open forum, and you don't want your users to have to select it no matter how they enter the node creation form. Are there any other scenarios?

And, just to confirm, has anybody encountered problems with forum vocabularies that are NOT hidden by taxonomy defaults? My testing has shown that it is only a problem when the forum vocabulary IS hidden, but I'd appreciate confirmation from other users.

I didn't put hands in it no more since that time, anyway my scenario is multiple open forums, no need to set a default term since regular users can enter edit form only from within a forum, and in this case forum module does the work of auto assign a default term.
Forums vocab is hidden for regular users and displayed to forum admins (via taxonomy role), in either case and regardless of how you enter the edit form, if taxonomy defaults is enabled (and if no default term is set for forum, of course) no forum is set for the topic, so it "disappears" from listings.

Making headway. Just closed #982614: Module breaks forum behavior of autoassigning forum taxonomy to new forum post as a duplicate of this issue. Working on a more elegant fix than the one in comment #4.

I've committed a fix for this and believe it is solid, but I would appreciate some testing. I've attached the patch if you just want to play with that, or you can get the latest -dev version.

I'll roll this into version 6.x-2.1 soon, but want to fix up a few other issues first.

Thanks Brad.

I tested 6.x-2.x-dev and it has solved the new forum node problem.

I filed a small UI bug and I'm having another issue related to taxonomies that require at least one term where the form fails to validate if the vocab is hidden. The default term doesn't satisfy the requirement. I'll try to boil it down and submit another issue this weekend. My workaround for now is to remove "Required" from the vocabulary.

Marking as fixed thanks to Brian's feedback. The other issues he mentioned are #989068: Broken Link in Warning and #810830: An illegal choice has been detected - please contact the site administrator.