There is not any open_basedir setting in any /etc/php5/*/php.ini config file.
If I comment out php5 and comment fcgi php in vhost file of ispconfig web interface on port 8080, I will get :
[Fri Mar 01 17:25:52 2013] [error] [client 188.8.131.52] PHP Warning: require_once(/usr/local/ispconfig/interface/lib/config.inc.php): failed to open stream: Permission denied in /usr/local/ispconfig/interface/web/index.php on line 31
[Fri Mar 01 17:25:52 2013] [error] [client 184.108.40.206] PHP Fatal error: require_once(): Failed opening required '../lib/config.inc.php' (include_path='.:/usr/share/php:/usr/share/pear') in /usr/local/ispconfig/interface/web/index.php on line 31
So there is also problem with permissions, but this is not related to fcgi setup I think..