I ran into a question from a client, who was working with FBA. He wanted to simplify his portal for extranet users, so he wanted to display a log out link on his page.
It turns out the solution is really simple: just create a link to the _layouts/signout.aspx page in a content editor webpart.
For you really lazy people, copy this url:
<a href="_layouts/signout.aspx">log out</a>
If you click on that link, you will be signed off. Thanks to Jeroen Van Bastelaere