Using Win2k or XP, IDLE 1.0.5 and Python 2.3.5, the
attached source produces a fatal error and IDLE and the
Python Shell are closed.

In order to reproduce the issue, start IDLE, open the
attached file and press "F5".

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Does this still happen with 2.4.1 and the corresponding IDLE?  
Besides numerous bug fixes, I believe 2.4 changed the 
interpretation of unsigned hex constants from sometimes 
negative to always positive.  From a glance at your file, this 
seems likely relevant.

If you also find a bug with current Python/IDLE, please reduce 
your file to the minimum necessary to produce the bug.  Also 
include the error message and traceback you get.

(As far as I know, there is currently no prospect of a 2.3.6, but 
there should at least be a 2.4.2)

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No, I had tested it also with 2.4.x and had no problem.

I didn't know whether there would be another 2.3.x, so I
submitted it.  In case the 2.3 branch is closed, do the same
with this bug.  ;)

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Oops, I think I should add my further findings nevertheless:
It was indeed the hex constants and their implicit
conversion to signed values.  An attached "L" solved the
problem.

Could it be that the warning messages cause possible errors
with IDLE on Windows?  With Linux, I see those "future
warnings" on the console when I try to import my file.  With
IDLE, these things are supressed, of course.  2.4 doesn't
show warnings, too.

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Probably not worth fixing in 2.3 branch at this point.