On a virtual hosting server I have the open_basedir set to .:/path/to/vhost/web:/tmp:/usr/share/pear for each virtual host. I have a client who's running WordPress and he's complaining about open_basedir errors thus:
PHP WARNING: file_exists() [function.file-exists]: open_basedir restriction in effect. File(/var/lib/php/session/sess_42k7jn3vjenj43g3njorrnrmf2) is not within the allowed path(s): (.:/path/to/vhost/web:/tmp:/usr/share/pear)
So the PHP session save_path isn't included in open_basedir but sessions across all sites on the server seems to be working fine apart from in this intermittent instance. I thought that perhaps the default session handler ignored open_basedir and this warning was caused by WP accessing the session file directly.
However from what I can see PHP 5.2.4 introduced open_basedir checking to the session.save_path config: http://www.php.net/ChangeLog-5.php#5.2.4 (I am on PHP 5.2.13).