And it looks like I'll be able to take advantage of Single Table Inheritance for the three similar models (News, Journals, ForumThreads), and at the same time take advantage of the Polymorphic Association with :as => :commentable for all the various models that accept comments.
NICE
Yep, I found a bug. STI and Polymorphic Associations don't play together nicely all the time. nzkoz (rails core member) says it's a real live defect.
Details here http://rafb.net/paste/results/cVQNiO28.html
Ok, not a bug. Whew. This is good news.