private the jforum start page as 'src' value for the iframe object. keep in mind though that closing the iframe would not automatically sign off the user ... (in case you use sso)
In this case you could call the "log off" action to jforum and send the respons in an invisible target [originally posted on jforum.net by Sid]
The longest recorded flight time of a chicken is 13 seconds. But that was done without this tiny ad: