I toyed with the idea of using reddit's open-source software to power a site. Stopped after reading this guy's rant about it.
I had the same problem using vBulletin: a constant stream of published exploits that needed to be patched. Every patch consuming time and money to migrate my customizations to the new code. (A stock site would be fairly simple to update.) I look forward to google providing a hosted discussion board product. Even if I can't mod it at least it'll be free and hosted.