WordPress - yes but not currectly with the setup we have here at WP.com. (We run different software here called WordPress MultiUser)
This has been a rumored feature planned for the future and hooks for it appear in the source code but nothing has been offically announced. Only method we would have here would be to suggest to you to (un)check the box that says if you want your blog to appear in search engines and ask to have no one link to it elsewhere.
Either that or grab a copy of the WordPress software and host it elsewhere where you can password protect your site.