The carrier has always been shown there on my phone (UK - O2 network). When you click on it it goes to a screen giving details about it - there's no way to change carriers, just provision for other carriers, like on every other phone.=)My phone now lets me select my carrier.
I didn't have to change anything. Just popped up with the 3.1 update....
(I have modified my carrier settings by visiting help.benm.at, but otherwise haven't jailbroken or unlocked. This showed up on the iPhone 32 GB 3Gs.)
Not a biggie.