diff -r 92455c1d6f83 -r b8d750daadde mercurial/hook.py
--- a/mercurial/hook.py	Sun Jan 18 19:59:51 2009 +0100
+++ b/mercurial/hook.py	Mon Feb 16 19:35:07 2009 -0600
@@ -70,7 +70,13 @@
 
 def _exthook(ui, repo, name, cmd, args, throw):
     ui.note(_("running hook %s: %s\n") % (name, cmd))
-    env = dict([('HG_' + k.upper(), v) for k, v in args.iteritems()])
+
+    env = {}
+    for k, v in args.iteritems():
+        if callable(v):
+            v = v()
+        env['HG_' + k.upper()] = v
+
     if repo:
         cwd = repo.root
     else: