diff -r c3b487048a0f Lib/threading.py
--- a/Lib/threading.py	Wed Jun 29 22:53:19 2011 -0500
+++ b/Lib/threading.py	Thu Jun 30 16:27:00 2011 +0200
@@ -1057,6 +1057,10 @@
     new_active = {}
     current = current_thread()
     with _active_limbo_lock:
+        if len(_active) > 1:
+            import warnings
+            warnings.warn("Calling fork() with multiple threads can cause "
+                    "deadlocks.", RuntimeWarning)
         for thread in _active.values():
             if thread is current:
                 # There is only one active thread. We reset the ident to