Refresh and login

Hello,
I've got a silverlight 5 application that at the startsup shows a loginform (similar to the TempHire application),authentication works fine and children are happy :D

I've got a small problem that's when the user press F5 the browser refresh and it asks me again to login... I've tried to check at the refresh if the Principal is logged in but it's false... how can I handle this scenario?

Thanks

Paolo

On my Shell view I do

  protected override void OnViewLoaded(object view)
        {
            base.OnViewLoaded(view);

            if (!userService.IsLoggedIn) //<- always false
            {
                PerformLogin().ToSequentialResult().Execute();
            }
        }

Thanks