I suspect that there is some deeper (more severe) issue behind the
problem I describe below. I've observed the following on Windows only.
Didn't try it with different OSs

running dragbug.py shows different behaviour with Python 3.1 compared to
Python 2.6:

Running it with Python 3.1 and performing heavy turtle dragging with the
mouse results in:

Fatal Python error: Cannot recover from stack overflow
This application has requested the Runtime to terminate it in an unusual
way. Please contact the application's support team
-- As I do at the moment ;-) 

Running the same script from Python 2.6: The error is much harder to
reproduce (only with very excessive dragging). It's a bit easier to
reproduce when running the program from a console and  if it occurs one
gets a conventional Python error message:

First a long sequence of:
Exception in Tkinter callback
Traceback (most recent call last):
Exception RuntimeError: 'maximum recursion depth exceeded while .... 
ignored

Followed by:

Traceback (most recent call last):
  File "dragbug.py", line 10, in <module>
    mainloop()
  ....
  File "c:\Python26\lib\traceback.py", line 57, in print_tb
    if hasattr(sys, 'tracebacklimit'):
AttributeError: 'module' object has no attribute 'tracebacklimit'

This problem can be overcome by setting a higher recurson limit. But I
think one should require that it doesn't produce a Fatal Error like in
Python 3.1

Regards,
Gregor