Reference: http://www.howtoforge.com/forums/showthread.php?t=53938

1) Added DEVSYSTEM define in config.inc.php.master  defaults to 0
  If set to 1 classes can load as symbolc links, server.php now

2) server.php now resolves its require()ed files with an absolute
   path instead of relative.

install/tpl/config.inc.php.master

@@ -57,6 +57,7 @@ $revision = str_replace(array('Revision:','$',' '), '', $svn_revision);
 //** Application
 define('ISPC_APP_TITLE', 'ISPConfig');
 define('ISPC_APP_VERSION', '3.0.3.3');
+define('DEVSYSTEM', 0);
 
 
 //** Database

server/lib/app.inc.php

@@ -75,7 +75,7 @@ class app {
 		if(is_array($cl)) {
 			foreach($cl as $classname) {
 				if(!@is_object($this->$classname)) {
-					if(is_file($conf['classpath'].'/'.$classname.'.inc.php') && !is_link($conf['classpath'].'/'.$classname.'.inc.php')) {
+					if(is_file($conf['classpath'].'/'.$classname.'.inc.php') && (DEVSYSTEM ||  !is_link($conf['classpath'].'/'.$classname.'.inc.php'))) {
 						include_once($conf['classpath'].'/'.$classname.'.inc.php');
 						$this->$classname = new $classname;
 					}
@@ -91,7 +91,7 @@ class app {
 		$cl = explode(',',$classes);
 		if(is_array($cl)) {
 			foreach($cl as $classname) {
-				if(is_file($conf['classpath'].'/'.$classname.'.inc.php') && !is_link($conf['classpath'].'/'.$classname.'.inc.php')) {
+				if(is_file($conf['classpath'].'/'.$classname.'.inc.php') && (DEVSYSTEM || !is_link($conf['classpath'].'/'.$classname.'.inc.php'))) {
 					include_once($conf['classpath'].'/'.$classname.'.inc.php');
 				} else {
 					die('Unable to load: '.$conf['classpath'].'/'.$classname.'.inc.php');

server/server.php

 <?php
-
 /*
   Copyright (c) 2007-2011, Till Brehm, projektfarm Gmbh
   All rights reserved.
@@ -28,8 +27,11 @@
   EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-require('lib/config.inc.php');
-require('lib/app.inc.php');
+// $script_path allows development work of using a symbolic link farm 
+// to use along side git or svn
+$script_path = dirname($_SERVER["SCRIPT_FILENAME"]);
+require("$script_path/lib/config.inc.php");
+require("$script_path/lib/app.inc.php");
 
 set_time_limit(0);
 ini_set('error_reporting', E_ALL & ~E_NOTICE);