I'm not saying that I really emphasize with AT&T, but I do generally agree that when you have a limited resource and people are charged for "unlimited" then eventually you will run out of that thing. When I first moved to Colorado, water wasn't metered in my neighborhood. Without meters, there's really no incentive to do anything more than use as much as you want. That's fine until we had a dry year and then another one after that. They can put up signs and advertisements and ask people to conserve, but there's always a few people - or more than a few - who just ignore that. Once they put in meters and tiered pricing, you could see a noticeable change in behavior.
I'm not saying that I like paying more for something - of course, I want things as cheap as possible - but I am saying that if you want to change behavior, tiered pricing is a very effective way to do it, and if you have a fixed amount of something then "unlimited" service doesn't encourage people to use anything less than as much as they want.
I don't use WiFi with my iPhone at home - 3G is plenty fast, and I always forget to turn off WiFi when I leave home and then my battery life isn't as good when I'm out of the house. But there's absolutely no incentive for me not to use as much data as I want. When I was in Europe, I pretty much never used 3G when I was home... as soon as I walked in the door, I switched off 3G and onto WiFi.
All that said, I don't have much sympathy for AT&T. As everyone has said they are milking the iPhone for as much as they can. $30/month for a data plan for my sister and mother who both use their iPhone data to check email and maybe the weather and that's about it and who maybe use 5MB each per month total is crazy. It's like SMS messaging at $0.20/message... it's completely divorced from the reality of what it costs AT&T to provide it. AT&T makes ~$3 billion in profit per quarter (on revenue of $30 billion with about a 50/50 revenue mix between wired and wireless services). $12 billion in profit per year doesn't make me feel much empathy for their poor situation.
They want to roll out tiered service - fine... but the iPhone is at the absolute edge of what I'm willing to pay for it. If they raise prices on it at all, and I'll just jailbreak, move it to T-Mobile and will happily cut data out of my plan - WiFi at work and at home... I'm mostly set. As it is, I sometimes regret that I bought a new 3GS when I look at my cell phone bill compared to what it was pre-iPhone 3GS on T-Mobile.
And clearly I'm not the only one who feels this way - most this thread is people complaining they feel like they are barely getting adequate value for their money, and my co-workers and friends say the same thing. Having a customer base of people where a substantial portion feel that they are paying too much for something shouldn't leave AT&T in the situation where they think they can raise prices much.