This site uses Plone 2.5 with patches. Here are some:

--- CMFPlone/DiscussionTool.py-backup   2007-04-07 13:03:50.000000000 +0530
+++ CMFPlone/DiscussionTool.py  2007-04-07 13:25:14.000000000 +0530
@@ -12,6 +12,8 @@
 from Products.CMFCore.interfaces.Discussions \
         import DiscussionResponse as IDiscussionResponse
 
+from Products.CMFCore.utils import getToolByName
+
 class DiscussionTool(PloneBaseTool, BaseTool):
 
     meta_type = ToolNames.DiscussionTool
@@ -48,7 +50,10 @@
             reply.text = reply.cooked_text = text
         elif text_format == 'plain':
             reply.text = text
-            reply.cooked_text = html_quote(text).replace('\n','<br>')
+            #reply.cooked_text = html_quote(text).replace('\n','<br>')
+            pt = getToolByName(self, 'portal_transforms')
+           reply.cooked_text = pt.convertTo('text/html', text,
+               mimetype='text/x-web-intelligent').getData()
         else:
             reply.cooked_text = HTML(text=text, level=level)
             reply.text = text