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I just submitted a bug report ([ 710041 ] sys.stdout IOError 
under Windows) with this same problem, so I closed it and will 
add some of my comments here, in the hopes they will be 
useful:

Python 2.1.2 (#31, Jan 15 2002, 17:28:11) [MSC 32 bit 
(Intel)] on win32.
Under Windoze 98, in a non-console application.

Calls to the write() methods seem to discard  the data, which 
is what I want (when running a cross-platform GUI program 
with debug print statements).

BUT, when the write() method is called with more than 
about 4KB, it raises "IOError: [Errno 9] Bad file descriptor", 
the same as if you had manually called the flush() method.

This IOError happens when write()ing even one character 
if "pythonw.exe -u" was used to start the GUI.

I work around it by defining my own bit-bucket for 
sys.std[out|err].
(NOTE: This bug was hard to track down!)