New Install:
$ mkdir blog
$ cd blog
$ svn co http://core.svn.wordpress.org/tags/3.0 .
Updating to a New Stable Version:
$ cd blog
$ svn sw http://core.svn.wordpress.org/tags/3.0/ .
Relocating from the old svn.automattic.com repository
$ svn switch --relocate http://svn.automattic.com/wordpress/tags/2.7.1 http://core.svn.wordpress.org/tags/2.7.1/
Installing/Updating WordPress with Subversion « WordPress Codex