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Yep, this bit me, too.

This bug is caused by:

http://sourceforge.net/tracker/index.php?func=detail&aid=436259&group_id=5470&atid=105470
[ 436259 ] [Windows] exec*/spawn* problem with spaces in args

As that bug notes, distutils already works around this. 
Idle could borrow the same workaround, except with P_NOWAIT.

Hey, maybe the workaround could be polished and published as
"os.spawn(executable, args, env=None, wait=False,
search_path=False)".  Neat.

Cheers, and thanks for Python.