--- a/mercurial/subrepo.py	Wed Apr 18 21:27:35 2012 -0700
+++ b/mercurial/subrepo.py	Thu Apr 19 23:36:42 2012 +0200
@@ -369,6 +369,8 @@
         return []
 
     def revert(self, ui, substate, *pats, **opts):
+        ui.warn('%s: reverting %s subrepos is unsupported\n' \
+            % (substate[0], substate[2]))
         return []
 
 class hgsubrepo(abstractsubrepo):