BuddyPress.org

This is nothing to do with BuddyPress, hackers can just try logging in as admin on your login form and find the same thing.

Number one thing to do when you first install WordPress is to change the admin account to something else. WordPress 3.0 will allow you to change this when you first install.

You can edit the admin username in your database in the wp_users table. Change the user_login and user_nicename value.