Rename files to titlecased
I strongly oppose testing for file existence. It introduces a race condition: the path may exist at the time of test and disappear by the time of use; yes I am paranoid. Besides, it really impedes dry runs/unit testing - to try an interesting case you need to actually create a file. The pythonic way is to ask forgiveness, not permission:

try:
    os.rename(old_path, new_path)
except OSError:
    pass # or an error message


Notice that it would also take care of other failures of os.rename, such as new_path already exists or is too long; the directory is read-only, etc.
I am also not sure that "sanitization" is to be done unconditionally. A command line option would be nice. 
Typically a debugging/diagnostic messages are sent to stderr. A quiet (or verbose) option is also desirable.
fun doesn't strike as a particularly good name.