Py2.6: idlelib/EditorWindow.py

change line 110 from: width=self.width,
to: width=self.width, insertofftime=0,

Py3.0
apply the same change to line 112

This will switch off cursor blink (and annoy people who want cursor
blink). So really instead of setting it to 0 you should set it to a
variable. On Windows the Win32 API has a function you can call to find
out the user's preferred blink rate and you can use that. On Mac OS X
there's no such thing so you would need to add an option to IDLE's
configuration dialog. On Linux the .Xdefaults file should be read and
*insertOffTime respected. But since for Mac you have to add it to the
configuration dialog, it might just be easier to just do that for all
platform a simple [ ] blinking cursor check box would suffice.