diff --git a/bitbake/lib/bb/main.py b/bitbake/lib/bb/main.py
index 910cd64444..8762f7220a 100755
--- a/bitbake/lib/bb/main.py
+++ b/bitbake/lib/bb/main.py
@@ -41,10 +41,35 @@ logger = logging.getLogger("BitBake")
 class BBMainException(Exception):
     pass
 
+def present_options(optionlist):
+    if len(optionlist) > 1:
+        return ' or '.join([', '.join(optionlist[:-1]), optionlist[-1]])
+    else:
+        return optionlist[0]
+
+class BitbakeHelpFormatter(optparse.IndentedHelpFormatter):
+    def format_option(self, option):
+        # We need to do this here rather than in the text we supply to
+        # add_option() because we don't want to call list_extension_modules()
+        # on every execution (since it imports all of the modules)
+        # Note also that we modify option.help rather than the returned text
+        # - this is so that we don't have to re-format the text ourselves
+        if option.dest == 'ui':
+            valid_uis = list_extension_modules(bb.ui, 'main')
+            option.help = option.help.replace('@CHOICES@', present_options(valid_uis))
+        elif option.dest == 'servertype':
+            valid_server_types = list_extension_modules(bb.server, 'BitBakeServer')
+            option.help = option.help.replace('@CHOICES@', present_options(valid_server_types))
+
+        return optparse.IndentedHelpFormatter.format_option(self, option)
+
 def list_extension_modules(pkg, checkattr):
     """
     Lists extension modules in a specific Python package
-    (e.g. UIs, servers)
+    (e.g. UIs, servers). NOTE: Calling this function will import all of the
+    submodules of the specified module in order to check for the specified
+    attribute; this can have unusual side-effects. As a result, this should
+    only be called when displaying help text or error messages.
     Parameters:
         pkg: previously imported Python package to list
         checkattr: attribute to look for in module to determine if it's valid
@@ -60,7 +85,7 @@ def list_extension_modules(pkg, checkattr):
             continue
         try:
             module = __import__(pkg.__name__, fromlist=[modulename])
-        except (ImportError, SystemExit, RuntimeError):
+        except:
             # If we can't import it, it's not valid
             continue
         module_if = getattr(module, modulename)
@@ -70,7 +95,7 @@ def list_extension_modules(pkg, checkattr):
             modules.append(modulename)
     return modules
 
-def import_extension_module(pkg, modulename):
+def import_extension_module(pkg, modulename, checkattr):
     try:
         # Dynamically load the UI based on the ui name. Although we
         # suggest a fixed set this allows you to have flexibility in which
@@ -78,7 +103,7 @@ def import_extension_module(pkg, modulename):
         module = __import__(pkg.__name__, fromlist = [modulename])
         return getattr(module, modulename)
     except AttributeError:
-        raise BBMainException("FATAL: Unable to import extension module %s from %s" % (modulename, pkg.__name__))
+        raise BBMainException('FATAL: Unable to import extension module "%s" from %s. Valid extension modules: %s' % (modulename, pkg.__name__, present_options(list_extension_modules(pkg, checkattr))))
 
 
 # Display bitbake/OE warnings via the BitBake.Warnings logger, ignoring others"""
@@ -104,6 +129,7 @@ class BitBakeConfigParameters(cookerdata.ConfigParameters):
 
     def parseCommandLine(self, argv=sys.argv):
         parser = optparse.OptionParser(
+            formatter = BitbakeHelpFormatter(),
             version = "BitBake Build Tool Core version %s" % bb.__version__,
             usage = """%prog [options] [recipename/target recipe:do_task ...]
 
@@ -168,24 +194,14 @@ class BitBakeConfigParameters(cookerdata.ConfigParameters):
         parser.add_option("-P", "--profile", help = "Profile the command and save reports.",
                    action = "store_true", dest = "profile", default = False)
 
-        def present_options(optionlist):
-            if len(optionlist) > 1:
-                return ' or '.join([', '.join(optionlist[:-1]), optionlist[-1]])
-            else:
-                return optionlist[0]
-
         env_ui = os.environ.get('BITBAKE_UI', None)
-        valid_uis = list_extension_modules(bb.ui, 'main')
         default_ui = env_ui or 'knotty'
-        if env_ui and not env_ui in valid_uis:
-            raise BBMainException('Invalid UI "%s" specified in BITBAKE_UI environment variable - valid choices: %s' % (env_ui, present_options(valid_uis)))
-        elif not default_ui in valid_uis:
-            raise BBMainException('Default UI "%s" could not be found')
-        parser.add_option("-u", "--ui", help = "The user interface to use (%s - default %%default)." % present_options(valid_uis),
-                   action="store", dest="ui", type="choice", choices=valid_uis, default=default_ui)
+        # @CHOICES@ is substituted out by BitbakeHelpFormatter above
+        parser.add_option("-u", "--ui", help = "The user interface to use (@CHOICES@ - default %default).",
+                   action="store", dest="ui", default=default_ui)
 
-        valid_server_types = list_extension_modules(bb.server, 'BitBakeServer')
-        parser.add_option("-t", "--servertype", help = "Choose which server type to use (%s - default %%default)." % present_options(valid_server_types),
+        # @CHOICES@ is substituted out by BitbakeHelpFormatter above
+        parser.add_option("-t", "--servertype", help = "Choose which server type to use (@CHOICES@ - default %default).",
                    action = "store", dest = "servertype", default = "process")
 
         parser.add_option("", "--token", help = "Specify the connection token to be used when connecting to a remote server.",
@@ -315,8 +331,8 @@ def bitbake_main(configParams, configuration):
 
     configuration.setConfigParameters(configParams)
 
-    ui_module = import_extension_module(bb.ui, configParams.ui)
-    servermodule = import_extension_module(bb.server, configParams.servertype)
+    ui_module = import_extension_module(bb.ui, configParams.ui, 'main')
+    servermodule = import_extension_module(bb.server, configParams.servertype, 'BitBakeServer')
 
     if configParams.server_only:
         if configParams.servertype != "xmlrpc":