CS4 or CS5 along with OSX 10.5 or 10.6 have a problem doing grayscale.
What happens is that no matter what selections you make in PS the OS always
converts whatever you send it to a fixed profile. In 10.5 its
Generic Gray which
is a gamma 1.8 profile. In 10.6 its a new profile called Generic Gray Gamma 2.2
which you’d think was a gamma 2.2 curve but its not. The little glitch you see
in the graphs is exactly the conversion of Gray Gamma 2.2 to Generic GG 2.2.

Using CS3 is the easiest workaround currently.