Index: config.py
===================================================================
--- config.py	(revision 1952)
+++ config.py	(working copy)
@@ -20,6 +20,7 @@
 import copy
 
 from twisted.python import log
+from twisted.python.reflect import namedClass
 
 from twistedcaldav.py.plistlib import readPlist
 
@@ -198,11 +199,31 @@
                 newParams = items["DirectoryService"].get("params", {})
                 _mergeData(oldParams, newParams)
             else:
+                # Lookup default parameters and insert them in the module
+                # variable if they exist. This process also ensures that the
+                # specified directory service class can be imported and that
+                # we have default parameters for the subsequent logic that 
+                # follows.
+                try:
+                    serviceClass = namedClass(dsType)
+                    if hasattr(serviceClass, "defaultParams"):
+                        serviceDefaultParams[dsType] = serviceClass.defaultParams
+                    elif dsType not in serviceDefaultParams:
+                        raise ConfigurationError(
+                            "Default service parameters must be specified in " \
+                            "%s using the 'defaultParams' class variable or " \
+                            "via the 'defaultServiceParams' module variable " \
+                            "in twistedcaldav.config." % \
+                            dsType)
+                except (ImportError, AttributeError):
+                    raise ConfigurationError("Unable to load directory service %s" % dsType)
+
                 if dsType in serviceDefaultParams:
                     self._data["DirectoryService"]["params"] = copy.deepcopy(serviceDefaultParams[dsType])
                 else:
                     self._data["DirectoryService"]["params"] = {}
 
+
         for param in items.get("DirectoryService", {}).get("params", {}):
             if param not in serviceDefaultParams[dsType]:
                 raise ConfigurationError("Parameter %s is not supported by service %s" % (param, dsType))