Hello, i have found 2 bugs:

1) When the captcha appears you click on it to change/refresh the captcha. In firefox nothing ocurrs and in opera appears:

<?xml version="1.0" standalone="yes"?><post><id>captcha</id><captcha><![CDATA[<a title="clic to reload a new image" href='javascript:JOSC_reloadCaptcha()'><img src="http://www.hosting.compara.org//components/com_comment/joscomment/captcha.php?refid=3d11758e5cf726d386186464fcd4f4c2"; alt="Security Image" />
<input type="hidden" name="security_refid" value="3d11758e5cf726d386186464fcd4f4c2" /></a>]]></captcha><noerror>1</noerror></post>

2) After post a comment, the page does not show the comment. You have to force a page refresh to view it.

I have joomla 1.5.10 and !JoomlaComment 4.0alpha3

Thanks on advance