BuddyPress.org

You probably already tried this tutorial:
http://www.michiknows.com/2007/02/12/who-else-wants-to-hide-their-wordpress-admin-folder/

The other technique is hacking WP core files:
http://www.socialblogr.com/2009/09/how-to-change-folder-name-on-wordpress.html

You could also try applying a filter to “admin_url”, but when I tried this awhile back, it didn’t work:
http://stackoverflow.com/questions/2584706/wordpress-where-the-admin-url-is-set

I should note that I’ve never done this and strongly advise you not to as well.

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WP should have a defined WP_ADMIN_URL variable similar to WP_PLUGIN_URL and WP_CONTENT_URL, but it doesn’t.