=== modified file 'bzrlib/builtins.py'
--- bzrlib/builtins.py	2008-10-12 16:11:17 +0000
+++ bzrlib/builtins.py	2008-10-25 20:58:40 +0000
@@ -1607,6 +1607,11 @@
             raise errors.BzrCommandError('bzr diff --revision takes exactly'
                                          ' one or two revision specifiers')
 
+        if using is None:
+            using = config.GlobalConfig().get_diff_tool()
+        if using == 'bzr':
+            using = None
+
         old_tree, new_tree, specific_files, extra_trees = \
                 _get_trees_to_diff(file_list, revision, old, new)
         return show_diff_trees(old_tree, new_tree, sys.stdout, 

=== modified file 'bzrlib/config.py'
--- bzrlib/config.py	2008-10-16 18:30:32 +0000
+++ bzrlib/config.py	2008-10-25 20:55:11 +0000
@@ -146,7 +146,15 @@
     """A configuration policy - what username, editor, gpg needs etc."""
 
     def get_editor(self):
-        """Get the users pop up editor."""
+        """Get the user's pop up editor."""
+        raise NotImplementedError
+
+    def get_diff_tool(self):
+        """Get the user's default diff program."""
+        raise NotImplementedError
+    
+    def get_merge_tool(self):
+        """Get the user's default merge program."""
         raise NotImplementedError
 
     def get_mail_client(self):
@@ -428,6 +436,12 @@
     def get_editor(self):
         return self._get_user_option('editor')
 
+    def get_merge_tool(self):
+        return self._get_user_option('merge_tool')
+
+    def get_diff_tool(self):
+        return self._get_user_option('diff_tool')
+
     def __init__(self):
         super(GlobalConfig, self).__init__(config_filename)