You can add "external" SVN Repositories (in)to a "local" one. For Example, you can have the Bleeding Version of Wordpress and then add an external SVN for a Plugin as well.
Do this by setting the svn:external property with a (line-seperated) value of directory name and svn-repos-URLs pairs. This property should be set on a directory.
svn:externals akismet http://svn.wp-plugins.org/akismet/trunk breadcrumb-navxt http://svn.wp-plugins.org/breadcrumb-navxt/trunk
Example SVN:Externals Property on a Wordpress wp-content/plugins directory with the SVN-Version of Akismet and Breadcrumb NavXT.
While Updating the main wordpress directory, external sources are updated automatically as well.