I think everyone is missing a subtle point here. If I run "du -s a a/b" then there are in fact NOT two hard links to the same file (Linux doesn't even allow hard linked directories). Rather, there are two command line arguments pointing to two directories that are not mutually exclusive. This is a subtle but important difference and I think POSIX is being misinterpreted/misused here.
Personally, I find the new behavior to be counter intuitive and I have talked to several confused users about this change. I have also seen at least one script break. Also, as Eric noted, the du implementations in Solaris, OSX and AIX (all of which are POSIX compliant) give the same output for "du -s a a/b" and "du -s a; du -s a/b"
