This has already been fixed in 3.2:

Python 3.2b2 (py3k:87413, Dec 21 2010, 07:09:13) 
[GCC 4.1.2 20070626 (Red Hat 4.1.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> subprocess.Popen(['foo'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/root/python/py3k/Lib/subprocess.py", line 708, in __init__
    restore_signals, start_new_session)
  File "/root/python/py3k/Lib/subprocess.py", line 1323, in _execute_child
    raise child_exception_type(errno_num, err_msg)
OSError: [Errno 2] No such file or directory: 'foo'

It looks like it was changed by Benjamin in r86595. Not sure why that wasn't backported.