If what you're talking about is the webmail system, the change Joseph posted should allow you to view _properly-formed_ HTML messages.  One aspect of being properly formed is that the body has the proper MIME type, namely text/html.  However, for some reason (crappy clients, most likely), many HTML email messages are sent with the wrong MIME type.  IMP won't display those, as they're badly-formed to begin with.  Lookout, I understand, is less picky.