The comment is wrong, os.path.normcase should not return s.lower(), as the other issue notes the case sensitivity is filesystem dependent.  

Returning s.lower() will break code when dealing with paths that refer to an NFS share on a Linux server, or to a path on a local case sensitive file system.

IMHO The comment in posixpath should be changed to something like this:

# On MacOS X most but not all filesystems are case sensitive, which
# means that using normcase(p1) != normcase(p2) doesn't necessarily mean
# that the paths refer to different files.

The current comment gives the impression that a trivial should be applied, which isn't true.