AT&T would be wise to use whatever power it has over Apple to convince them to allow Opera Mini onto the iPhone App Store.
Not only will AT&T's customers have a much better experience when they don't have a perfect WiFi or 3G connection, but Opera Mini also compresses data up to 90%. This means that AT&T could get much more out of its current network infrastructure.
If 10% of their iPhone customers switched to Opera, they would be able to service quite a few new customers without upgrading their network capacity.
It should be noted that AT&T is already working with Opera on other projects, so they already know what Opera Mini is capable of.