BuddyPress.org

There is a great plugin,

http://wpmudev.org/project/New-Blog-Defaults

Updated for WPMU 2.7 – New features include the ability to delete the Mr. WordPress initial comment, the ability to place the initial Hello World post in draft mode (so it won’t show up on the site), a bunch of new discussion settings for 2.7 users, the default large image size, and more. Allows a site admin to set new blog defaults via an administrative screen in the site admin area.