Thanks for posting that, I was getting a bit confused.
I'm still slightly confused so just a couple of points I'd like confirmed. When the AF-on button is set for focus it also does metering, if you change the shutter button to metering only then this overrides the metering of the AF-on button when taking the shot?
Also, if you use the AE-lock button to meter off another part of the frame, the shutter button does not then override this setting when pressed (hence the word "lock"!).