Be careful for what you ask for....I'm also now firmly in the "bring on the tiered pricing" camp. I looked at our past 12 months of usage, and the most data my wife and I (combined) have used is 150MB. I'm paying $60/month for that. I love having it, because we use it all the time, but it's just email/web stuff, so the data is minimal. But I'm paying a ton per MB for it right now.
If at&t offered a 512mb 5 dollar a month plan, i could easily see the vast majority (90%) of iPhone subscribers switching to that. Which means AT&T would getting ~1/6th the money they received before. So they would need to add 6x the data subscribers they have now to recoup the costs. Now the bad part, i could actually see thing happening. If people are complaining about having 1000kbps less speed right now, imagine what having 6 times as many users would do?
The way that i see it is that high data rate is to keep the subscribers relatively low until the current wireless carriers can upgrade their networks to support larger bandwidth.