I am running OSX 10.5.8 on this macbook. The Tcl/Tk package on the system is ActiveState Tcl/Tk 8.4.19.
I just installed Python 3.2 (r32:88452, Feb 20 2011, 10:19:59) from http://www.python.org/getit/releases/3.2/ and I am still seeing this bug. It does not occur with the Python 2.7 Tkinter package, which uses the same Tcl/Tk framework. So if it is a bug in Tk, it is harmless when used with Python 2.7. But, after all, it is not so surprising to see new UnicodeDecodeErrors when porting from Python 2 to Python 3.