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I did some more investigating, and it turns out that Jim and Benjamin's 
analysis of the problem is correct. The distribution that most people install 
for MacOSX is a "batteries included" distribution of Tcl/Tk, which includes 
lots of optional packages that initialize themselves. Some of these 
initializations do a compare of tcl_version to the numeric value 8.3 or so, 
thereby coercing tcl_version to a float. I removed all traces of tcl/tk from my 
machine and installed a minimal tcl/tk and the problem does not occur.

On Linux everyone apparently installs a bare tcl/tk, but once you install 
incrTcl or whatever yourself you will run into the same problem.

As to your (Martin's) statement that _tkinter uses only official Tcl APIs: the 
TCL folks disagree with this. The official way to get a float from a tcl object 
is using Tcl_GetDoubleFromObj(), not looking at the cached internal value. A 
tcl object is a string, and that's all there is. Anything else is optimization.

The question now is really: how important is patch #518625. It apparently 
breaks Tcl object semantics, is the added convenience worth it. If it is then 
we simply add str() calls around the comparison of tcl_version in Tkinter, but 
then we should remember that this same problem will show up in different 
guises later.