I'm on OSX 10.8.
Before my experiment it's like this:
Nathan-der-Graue:~ max$ ls -l /bin/bash -r-xr-xr-x 1 root wheel 1333920 17 Feb 15:22 /bin/bash
Then I run
Nathan-der-Graue:~ max$ sudo chmod +s /bin/bash
I expected this to make bash always come up as a root shell, but it seems that it didn't change anything to bash's behaviour:
Nathan-der-Graue:~ max$ /bin/bash bash-3.2$ whoami max
I know this isn't a good idea at all. Where is my mistake? I guess I have misunderstood the +s-flag.