AT&T will auto-detect that you're using an iPhone under your blackberry plan and automatically switch you over, billing wise.
If you upgrade to 3.1.2, you need to make some choices. Like if you want to REMAIN able to unlock your phone to any GSM network other that AT&T. If you upgrade through iTunes, your iPhone will function perfectly, however you will lose your ability to go to other networks.
If you DO want to keep your ability to go to other GSM carriers, you can update outside of iTunes with a custom created ipsw (Firmware/Operating System) using the Pwnage Tool on the Mac.
Give me more details on what you're looking for, maybe I can help.