I believe we have figured out the issue, and I think it lies with IE8. There is a new setting within IE8's Delete Browsing History area (Internet Options > Browsing History Delete button) labeled 'Preserve Favorites Website Data'. If you delete your history with this box is checked, the login page will load exactly as you left it, i.e. if you were logged in you still are logged in, but if you were logged out you aren't auto-logged in. However, if you log out, and delete your history with this box unchecked, when you reload the homepage you are auto-logged in as expected. The description of this setting is that it will 'Keep cookies and temporary Internet files that enable your favorite websites to retain preferences and display faster.' Apparently the cookie being used for the authentication check isn't deleted unless you un-check this box. This is a new setting in IE and I only noticed it today.