The following behavior should be documented but it is not:

If the user has a .Idle.py file IDLE will run it when it starts up. This 
is independent of running IDLESTARTUP or PYTHONSTARTUP when the -s 
switch is given. It is run by Tk.readprofile as called from Tk.__init__. 
The "Idle" comes from the name passed to TK() when PyShell.py creates 
its Tk root. In fact, not only is it independent, but it works 
differently: any imports done in .Idle.py go into Tk's name space, 
whereas IDLESTARTUP/PYTHONSTARTUP is exec'd and imports go into the 
interpreter's namespace.

Note that the 'I' in '.Idle.py' is really uppercase, which will matter 
on case-sensitive platforms.