Fill the (entire) page with a login screen if the user is not connected, else add a little widget that can replace the meta one.
Mandatory Authentication is a plugin that put a WordPress blog behind a login form. It can be usefull for a family blog whose members do not want to make their blog accessible from visitor but do not really need to have a hight security. This plugin does not sweat to those who host confidential informations, use a plugin like wp-sentry instead !
It lets users login, and then redirects them back to the blog, it also shows error messages.
NOTE: it is based on sidebar-login version 2.1.4 from jolley_small, thanks to his great job !