See http://docs.python.org/dev/library/multiprocessing.html#multiprocessing-
programming

Specifically:
Joining processes that use queues

Bear in mind that a process that has put items in a queue will wait 
before terminating until all the buffered items are fed by the “feeder” 
thread to the underlying pipe. (The child process can call the 
Queue.cancel_join() method of the queue to avoid this behaviour.)

This means that whenever you use a queue you need to make sure that all 
items which have been put on the queue will eventually be removed before 
the process is joined. Otherwise you cannot be sure that processes which 
have put items on the queue will terminate. Remember also that non-
daemonic processes will be automatically be joined.