subprocess.py contains the following line (380 in CPython 2.6.3):
    mswindows = (sys.platform == "win32")

which only correctly detects CPython on Windows.  This line should be changed to:
    mswindows = (sys.platform == "win32" or sys.platform == "cli" or sys.platform == "silverlight")

so subprocess can be utilized from IronPython as well.

This bug should be trivial to fix.

Probably it should use platform.system() == 'Windows' instead.

but, does it work even with your fix?
it seems to me that "from _subprocess import *" will fail miserably.

Also, using platform.system() == 'Windows' would exclude IronPython running on Mono.

platform.system()=="Windows" won't work unless you change platform as well:
IronPython 2.6.1 DEBUG (2.6.10920.0) on .NET 2.0.50727.4927
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.system()
'cli'

it's not about platform detection; does IronPython have the equivalent of msvcrt.open_osfhandle()?

Is there any reason _subprocess couldn't be implemented in C#?  If not, this is the first of many steps needed to get subprocess working under IronPython.

Proposed patch, with duck subprocessing. :D

Amaury is right. On Windows, _subprocess is a built-in, so the platform stuff just currently enables further failure.

Dave: My C# skills aren't the greatest, but looking at PC/_subprocess.c there are a few Win32 functions you might need to P/Invoke, but I believe a lot of the functionality is supported in .NET.

Florent's patch makes sense to me.

What does the patch achieve?

Jeff Hardy just made this change for IronPython 2.7: http://bitbucket.org/ironpython/ironlanguages/changeset/b6bb2a9a7bc5

Any opposition to us matching that so they don't need to patch Lib/subprocess.py?