Sorry, I didn't fully comprehend the depth of your question.
I'm flying a little blind here, because I'm using GW6.0 and a really old version of Intellisynch on my own PC. I don't see any field in GroupWise Tasks for "category", so I'm guessing it's new to v6.5? If you're sure there's a field to put a category in GW6.5, then I'd open the Intellisynch software, choose the ToDo List config, make sure GroupWise 6.5 is selected as the synch target, click on "Advanced" button, then "Field Mapping" and make sure the Palm Category <-> GW Category mapping is correct. In my old version of Intellisynch there is no corresponding field available in GW for mapping to the Palm "Category" field that makes sense. You may need to upgrade to a newer version of Intellisynch to get the correct conduits.
Again, I'm assuming there is a new field in GW6.5 for categorizing tasks, as I do not see anything in GW6 that correlates to that ability in the the Palm software. If this is incorrect, then I have to believe the answer is there is not means to categorize ToDo's in Groupwise so you're stuck with the Unfiled category on the Palm.
The only other thought I have is to use another field in GW like Priority, which is limited to a single letter followed by a number (e.g. A1, B22, c17, etc). This would mean categorizing the entries on the Palm similarly, which I'm assuming would be useless to your user.
Not a lot of help, I know. Maybe someone wiser will help on this thread.....
Dennis