Quoting from msg179101 on #16823, which I turned into a doc issue:
'''
What you are doing appears to be unsupported (invalid). From
http://www.astro.washington.edu/users/rowen/TkinterSummary.html
"all Tkinter access must be from the main thread (or more precisely,
from the thread that calls the mainloop). Violating this is likely to
cause nasty and mysterious symptoms such as freezes and core dumps."
'''
You got the freeze part. I suspect what changed from 2.5 to 2.6 was the tcl/tk version delivered with Python on Windows.