While the original story cites "Cloned" themes, the specific example, TweetPress, isn't cloning Twitter per se. As I understand it, the theme is simply calling twitter's API to go get the specific elements allowed via the API and then build a theme that mimics the user's twitter page - for use as a WP theme. I think people, including the author, (read the comments - http://www.themelab.com/2010/01/31/wordpress-clone-themes-your-take/#comment-11978 ) Leland, originally missed what the theme was really doing.

As for the rest of the "cloned" themes mimicing other sites like FB, YT etc - that's a whole other story. :)