Source: http://codex.wordpress.org/Installing/Updating_WordPress_with_Subversion
To install a fresh copy of WordPress, type the following at your command prompt, starting from the directory above where you want to install WordPress. These commands will create a directory “blog” for your blog, and then check out (Subversion command “co”), or extract from the repository, the stable version of WordPress you have chosen. Do not type the “$” characters — they are just indications of the command prompt. Here are the commands (assuming the version you want is 2.3.1):
$ mkdir blog
$ cd blog
$ svn co http://svn.automattic.com/wordpress/tags/2.3.1 .