@till: Well it does not really help, because the php.ini settings are overwritten by the vhost.conf settings. Adding it to the httpd additional stuff won't help, because these values are written before the php stuff in the conf though they have no effect.
Is there a way to change that, to either always allow pear, make the user check "allow pear" or most flexible, an additional box for the admin to add pathes to openbasedir...
In my case I changed linke 1448 from config.lib.php
php_admin_value open_basedir "/usr/src/php/:".$mod->system->server_conf["server_path_httpd_root"]."/"."web".$web["doc_id"]."/\"