I noticed that these days __file__ attributes of modules are case normalized (ie. lowercased on case insensitive file systems), or at least the directory part. Then I noticed that this is caused by the fact that all sys.path entries are case normalized. It turns out that site.py does this, in a function called makepath(), added by Fred about 8 months ago.
I think this is wrong: we should always try to *preserve* case.
There is an added problem with the makepath() stuff that I hadn't reported here yet: it has broken MacPython on some non-western machines. Specifically I've had reports of people running a Japanese MacOS that things will break if they run Python from a pathname that has any non-7-bit-ascii characters in the name. Apparently normcase normalizes more than just ascii upper/lowercase letters.
And aside from that I fully agree with Just: seeing a stacktrace with all lowercase filenames is _very_ disconcerting.
I would disable the case-normalization for MacPython, except that I don't know whether it actually has a function. With MacPython's way of finding the initial sys.path contents we don't have the Windows-Python problem that we add the same directory 5 times (once in uppercase, once in lowercase, once in mixed case, once in mixed-case with / for \, etc:-), so if this is what it's trying to solve we can take it out easily.
I can't think of any function besides the attempt to avoid duplicates. I think that even on Windows, retaining case makes sense. I think that there's a way to avoid duplicates without case-folding everything. (E.g. use a case-folding comparison instead.) I wonder if maybe path entries should be normpath'd though? I'll leave it to Fred, Jack or Just to fix this. --Guido van Rossum (home page: http://www.python.org/~guido/)