There's no info on the allocation, but you can have a decent guess based on the demographic area of the store in question. For example, an AT&T store in a high-end mall that also has an Apple store will most likely have a LOT of iPhones to go around, as opposed to an AT&T store in the middle of nowhere in a bad neighborhood. But, I'm sure you're saying "duh!"
All I know is that there are a lot more AT&T stores than there are Apple stores. You could probably drive around to a FEW AT&T stores and find one with a decently short line. 🙂