Buffering problem using subprocess module

[Dr. Who]

I am using the subprocess module in 2.4.  Here's the fragment:

bufcaller.py:
	import sys, subprocess
	proc = subprocess.Popen('python bufcallee.py', bufsize=0, shell=True,
stdout=subprocess.PIPE)
	for line in proc.stdout:
		sys.stdout.write(line)

bufcallee.py:
	import time
	print 'START'
	time.sleep(10)
	print 'STOP'

Although the documentation says that the output should be unbuffered
(bufsize=0) the program (bufcaller) pauses for 10 seconds and then
prints START immediately followed by 'STOP' rather than pausing 10
seconds in between them.  Note that I made bufcallee a Python script
for ease of the example but in the real-world problem I am trying to
solve it is simply an executable.

Any ideas?
Jeff