Raymond Camden recently asked on his blog How can you timeout a session in an Ajax-based application?. Most of the comments on the entry relate to doing some kind of server-ping, but my suggestion would be to just let your application tell your Ajax code when the session actually expires.
The concept is to use HTTP response headers to inform your Ajax request when the session has actually expired. This allows you to do everything in one single call, without having to worry writing other code. Besides you should be adding some failure code to your Ajax calls anyway, so this fits right in with good Ajax practices.
There are two basic approaches you can take. Using a "custom" response header or sending back HTTP status code to indicate the user is no longer authorized to view content.