I don't agree with the notion that the other networks such as Verizon or Sprint would have had the same issues as ATT. Verizon, for example, has also seen a massive smartphone boom on their network, and they have upgraded their technology to cope with the increased workflow. That's why they are deploying LTE so quickly, because they expect the smartphone boom to continue to incline and cause more workload on the network. Another thing people have a lot of trouble realizing is the fact that the iPhone population isn't anything too crazy. The chunk of ATT's network that iPhones take up is not all that big. It's just sheer size of the population of people using the network. So this is not an iPhone specific problem. Verizon has plenty of customers who are grabbing data like crazy with their blackberries. I see more blackberries than I do iPhones where I live.
The point I'm trying to make is that the iPhone isn't the root of the problem with ATT right now. They simply aren't putting enough resources into building and maintaining their network to support the entire wireless ATT population.
I wonder how many latent future iPhone users are out there waiting until the iPhone hits other carriers. Surely this is a significant number.
Also, based on Flickr camera data, I believe that the iPhone significantly affect data usage. If the bandwidth were available, Apple would push (or at least customers) for more bandwidth intensive apps (think Over-The-Air use of Hulu, SlingBox, iTMS Movies).
Leisure usage trumps business usage for potential to saturate the network. I loved the email on my Blackberry when I had one, but it was a nightmare for consumer-based use.
You are right, though. It's not the fault of the iPhone, really, but, every carrier would want to limit the unlimited data plan (without losing a significant number of subscribers). It's in their best interest.