Spot the difference: 

Python 2.5.2 (r252:60911, Aug  6 2008, 09:17:29)
[GCC 4.3.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> def p(string):
...     print re.sub(r'\n +', '\n', string)
...
>>>
>>> import os
>>> p(os.popen2.__doc__)
Execute the shell command 'cmd' in a sub-process.  On UNIX, 'cmd'
may be a sequence, in which case arguments will be passed directly to
the program without shell intervention (as with os.spawnv()).  If 'cmd'
is a string it will be passed to the shell (as with os.system()). If
'bufsize' is specified, it sets the buffer size for the I/O pipes.  The
file objects (child_stdin, child_stdout) are returned.
>>>
>>> import popen2
>>> p(popen2.popen2.__doc__)
Execute the shell command 'cmd' in a sub-process. On UNIX, 'cmd' may
be a sequence, in which case arguments will be passed directly to the
program without shell intervention (as with os.spawnv()). If 'cmd' is a
string it will be passed to the shell (as with os.system()). If
'bufsize' is specified, it sets the buffer size for the I/O pipes. The
file objects (child_stdout, child_stdin) are returned.
>>>

It took me a while to realize why my program was failing since I was reading the documentation for one version and using the other :-)