[Zope] user log-ins not persisting

I have a situation where Zope log-ins only seem to be recognized by the first page viewed after the user logs in. I have only been able to duplicate this in Mozilla I'm using the following if-logic to check if a user is logged in and selectively display content. <dtml-if "AUTHENTICATED_USER.getUserName() == 'Anonymous User'"> For the first page after they authenticate this tests false, yet on subsequent pages it tests as true... Any Ideas on what's happening here? Much appreciated. -- David