I have a few internal .net web application here that require users to \"log out\" of them. I know this may seem moot on an Intranet application, but nonetheless it is there.
Why not use FormsAuth, but against ActiveDirectory instead as per the info in this thread. It's just as (in)secure as Basic Auth, but logging out is simply a matter of blanking a cookie (or rather, calling FormsAuthentication.SignOut)