At http://docs.python.org/lib/thread-objects.html, under join(), it says:

"As join() always returns None, you must call isAlive() to decide whether a timeout 
happened."

This would be better if it were more explicit, i.e.

"As join() always returns None, you must call isAlive() after calling join() to 
decide whether a timeout happened; a return value of True indicates the join() call 
timed out."