osx tkinter listbox problem

[Ben Collver]

I am using Darwin 10.4.9, tcl 8.4.7, tk 8.4, and python 2.3.5.  I have 
also tried Python 2.5 on Darwin, Debian, and Fedora Core 6.

I am working on a GUI front-end to a Python program of mine.  It is a 
simple grid with labels in the left column and input widgets in the 
right column.  One of the rows has a listbox in the right column.  The 
listbox contains two items.  I select one using widget.select_set(). 
The GUI is not complete yet, because I am still learning about Python 
and Tkinter.

On Linux, the GUI works as expected.  On OSX, when select_set() runs, 
the listbox disappears from its place in the grid, and appears in the 
upper left corner of the window.  Here are screen shots to illustrate.
http://terrorpin.net/~ben/docs/alt/music/soundfont/pysf-wip/pysfi-debian.jpg
http://terrorpin.net/~ben/docs/alt/music/soundfont/pysf-wip/pysfi-osx.png

The GUI code itself is at the following location.
http://terrorpin.net/~ben/docs/alt/music/soundfont/pysf-wip/pysfi.py

To run, it needs the Python program at the following location.
http://terrorpin.net/~ben/docs/alt/music/soundfont/pysf-wip/pysf.py

Why does the listbox change position at all?  Am I using listbox 
incorrectly?

Thank you,

Ben