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It's worse than I thought; there isn't even one setting for shell that works everywhere. This is what happens on POSIX (tested on Mac and Ubuntu):
$ mkdir /tmp/xxx
$ cd /tmp/xxx
xxx $ virtualenv /tmp/zzz
xxx $ python
Python 2.6.5 (r265:79063, Mar 23 2010, 08:10:08) 
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from subprocess import *
>>> p = Popen(['python', '-c', 'import sys;print sys.executable'], 
... stdin=PIPE,stdout=PIPE,stderr=PIPE,
... env={'PATH':'/tmp/zzz/bin'})
>>> stdout,stderr = p.communicate(None)
>>> print stdout
/tmp/zzz/bin/python
>>> print stderr
>>> p = Popen(['python', '-c', 'import sys;print sys.executable'], shell=True,
... stdin=PIPE,stdout=PIPE,stderr=PIPE,
... env={'PATH':'/tmp/zzz/bin'})
>>> stdout,stderr = p.communicate(None)
>>> print stdout
>>> print stderr