I have a Motorola Razr on AT&T, and it has always been fine for me.
What I've heard is that the iPhone's use of data networks, or perhaps the fact that an iPhone is a great way to interactively use the Web and so users use it a lot, means that if a typical "web capable" phone user uses 1 (one) megabyte of download, the typical iPhone user uses 400 MB of download each month. Or perhaps its the "average" that is 400 times more.
And in big cities, specifically NYC, LA and SF, the tech geeks have all been buying iPhones and using the heck out of them. So the city-wide usage may be a thousand or more times the load that they used to experience.
Scaling any network to handle 1000 times more usage is a hard problem.
My guess is that if AT&T delivered what the users wanted, it would take even more bandwidth. There are folks on the tech-TV and podcast world who rant against AT&T and recommend that fellow geeks not get an iPhone. Thus there could be even more bandwidth needed to sate the needs and wants.
Clearly, AT&T needs to put up a lot more towers and get more bits into the air. But that may be impossible.
Perhaps they should let Verizon or Sprint have the iPhone, and the headaches.
It wouldn't surprise me if Apple did start to ration 3G bandwidth. It already over-charges iPhone users for data ($15/mo more expensive than non-iPhone unlimited data plans) and messaging ($5/mo more expensive than non-iPhone unlimited messages plan). So yeah, they don't like iPhoners using their data plans, they structure the plans to make economically minded folks stay away from the iPhone. Now they just have to come up with a reasonable scheme to keep those who get one anyway from being able to use their 'premium data plan' (while not keeping them from paying for it).