I've been working on a new _tkinter (named it as "plumage") these days
and I hit this same problem for trusting too much that nothing from tcl,
including tk and extensions, would give me this embedded null.

Checking another bridge to Tcl (one done for Perl) it is possible to
notice that it also chose to verify for these bytes and convert them to
something else, a 0. The code for this for Python can be found at
http://code.google.com/p/plumage/source/browse/trunk/src/utils.c#42 up
to line 76, it could/should be adapted to the _tkinter in py3k and also
for python 2.x.