I'm currently developing a script that makes extensive use of threads 
and Popen, with threads being created dynamically and each thread 
creating a large number of Popen processes.  

If I limit the thread count to 2 (main + worker) then the problem 
appears to disappear (or is just intermittent) however if I run with 
more than 2 threads or from within winpdb then the dead lock occurres 
rather consistently (and in the case of winpdb, always)

According to winpdb the script hangs on line 1086 of subprocess.py 
(from 2.5.2), strangely all remaining worker threads hand at this 
point:
    # Wait for exec to fail or succeed; possibly raising exception
==> data = os.read(errpipe_read, 1048576) # Exceptions limited to 1 MB
    os.close(errpipe_read)
    if data != "":
        os.waitpid(self.pid, 0)
        child_exception = pickle.loads(data)
        raise child_exception

I tried the suggestion of adding close_fds=True or using a global lock 
but the script still hangs under winpdb.  A solution that did appear 
to work was having both a global lock and adding close_fds=True to the 
call list.  

Running the script under pdb or cProfile appears to fix the problem as 
well...

NOTE: winpdb appears to bring out the worst case scenario and reliably 
reproduces the problem.

This is running on FreeBSD 8-Current amd64 (from early August) with 2 
cores.