Ron,
It is NOT an OS update. It is only a 10k file and happens almost instantly. BUT...I do not know how it would affect a jailbroken phone's functionality. Also..to get it, you have to hit the "check for update" button in iTunes...which of course for you would also bring up the 3.1 regular update which would revert your jailbroken phone. So...it might be better to wait for the jailbreak community to incorporate the carrier settings update in their future release. Or at least wait until they define the procedure on how you would go about installing the carrier settings update, just to be safe. It should work with 3.0 and above though. Since every other iPhone in the world has this already in 3.0 except AT&T folks due to their network not being ready.