diff -r 06bc1ef248a6 -r 8ff4532376a4 mercurial/commands.py
--- a/mercurial/commands.py	Wed Jun 01 17:01:09 2005 -0800
+++ b/mercurial/commands.py	Wed Jun 01 19:19:38 2005 -0800
@@ -68,8 +68,13 @@
     # this should eventually support remote repos
     os.system("cp -al %s/.hg .hg" % path)
 
-def checkout(u, repo, changeset=None):
+def checkout(ui, repo, changeset=None):
     '''checkout a given changeset or the current tip'''
+    (c, a, d) = repo.diffdir(repo.root, repo.current)
+    if c:
+        ui.warn("aborting (outstanding changes in working directory)\n")
+        sys.exit(1)
+
     node = repo.changelog.tip()
     if changeset:
         node = repo.lookup(changeset)