I use Outlook 2003... and it just occurred to me when I read your post that Outlook could be my culprit and not Register Plus. Sure enough, I checked my GMail on the web and the link was there. I can only assume that my comcast webmail will also have a link, since I can't login to my account at the moment.
Do you know why it works fine in my Outlook if "Customize User Notification Email" is not enabled, but once I enable it I get a plain text "link"?
A reason this "solution" escaped me is because I turned my security settings as low as I could, yet I still had the problem.
...and FYI if you still want to know. I am using IE7, Fire Fox, and Windows XP SP2.
Thanks