If you need to display the login link on the portal after a user has logged out, Angela has posted how to do it here:
Save User Frustration by adding the magic ‘Log (back) In’ button
Thanks Shane for pointing everyone back to this blog post!
What’s really nice is that this one change affects all of the web applications, the Portal, Web Report Studio and the Stored Process Web App.
Also the sas-bi.blogspot.com link will be going down in the next couple months, the new blog post is at: http://blogs.sas.com/content/bi/2010/11/23/save-user-frustration-by-adding-the-magic-log-back-in-button/