As far as I know, iTunes won't let you install the carrier update without first upgrading your firmware to 3.1, which is fine for 3G phones, so anyone using a jailbreaked 3.0/3.0.1 3G S iPhone shouldn't and can't install the carrier update. They have to install their own carrier update.
There are many threads in this forum with instructions on how to do just that.
I have a 3GS and I have done it successfully.