I hope this isn't old news, but I tried searching and couldn't find anything.
I just bought a brand new *ist D a few weeks ago and couldn't figure out how to stop down the aperture for metering, for my manual lenses. My green button would only meter at wide open aperture. I was starting to think it was only a feature for the newer cameras.
As it turns out, it was just a matter of updating the firmware. I had version 1.00 and updated to v1.12 and it works like a charm now!
To check your firmware version, hold down Menu while turning the camera On.
For firmware updates for *ist D:
Software Downloads : Support & Service : PENTAX