Sergiu Dumitriu wrote ..
 Try this:
 {{html clean="false"}}
 <fieldset>
 <input type="hidden" name="classname"
value="XWiki.XWikiComments" />
 <textarea name="XWiki.XWikiComments_comment" rows="5"
cols="80"></textarea>
 <input type="submit" name="action_objectadd" value="Send
comment"/>
 </fieldset>
 {{/html}} 
 Fantastic.  Thanks.  I should have thought to look at the comments...
  The trick is the name of the submit button.
action_objectadd instructs
 the server to ignore the action specified in the URL where the form was
 submitted, and instead to use the action that's specified after the
 "action_" part, in this case "objectadd". This special form parameter
 gets submitted only when clicking the "Send comment" button. 
 Out of interest where is that caught, I'd like to take a look at the
 processing.  If you're pretty busy don't worry - I'll probably
 grep around in the files or code till I find it anyway!
 
core/xwiki-core/src/main/java/com/xpn/xwiki/web/ActionFilter.java
--
Sergiu Dumitriu