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I've tracked down some more info:

It also happens on XP using 2.4.1 and Idle 1.1.1.

The way to trigger it is to start IDLE with no subprocess. 
The way that I got that to happen is to right click on
Turtle.py in Windows Explorer, and select "Edit with IDLE".

While the fix that I suggested still cures it, it may be
possible to achieve a better general solution by leaving
turtle.py alone, and fixing IDLE...

-T.