This underConstruction plugin allows WordPress site developers to put up temporary page that site users who are not logged in will see.

You can use this to build a site live, that can only be viewed by users who are logged in, while non logged in visitors will get the ‘Under Construction’ page.

As of version 1.04, underConstruction supports WordPress 3.0-alpha release.

More Info at: http://masseltech.com/plugins/underconstruction/