Yes, I know it's functionless without a correctly and network enabled paired sim card, but I thought I'd post a followup on this to anyone who was interested in knowing more about it. Also note, this requires an activated (or hacktivated) iPhone.

The Carrier option can be seen by doing exactly this:
1. Turn off phone
2. Insert the original AT&T sim card that came with the phone
3. Turn on phone (yes, you need to do turn off the phone and turn it back on even if you already had the card installed)
4. Go to the phone option and dial 911
5. Hang up the instant you see the option to
6. Go to Settings, and you should see the Carrier option now.

If you tap it, it will load for a brief few seconds, then you will be presented with a list of the carriers available locally for use.

Anyway, that's it. You can't use it for anything, and it's pretty useless in general without an AT&T (or cingular etc) activated and working for calls sim.

Just posting it for curiousity's sake.