Correct me if I'm wrong since I'm not a US-based customer of Apple's products, and therefore haven't yet had to deal with AT&T.
You are paying AT&T $35 each month for them to provide a service (3G network) to a product they happen to sell exclusively and which is portrayed in commercials as being able to do such things as stream video, audio, or run rich content applications without a mention of being "limited" to WiFi or other kinds of networks.
Why is it that it's unreasonable to expect to be able to use the 3G network as advertised? I live in France, where Orange (the premier iPhone carrier) has had an unlimited plan since the beginning (or iPhone 3G launch, can't remember), and have excellent coverage. Sure, you can hook up to your WiFi network at work or at home if you feel like it, but if you do so, you're not only limiting the use of the phone (need to leave? well that download isn't following you anywhere, unlike on a 3G network), you're also paying much more than you need (why not buy into a much cheaper 1Gb plan?). I am paying for unlimited data. Not "unlimited data* (*over WiFi)". I don't use WiFi because it's not convenient for me (I don't have a home WiFi network nor a work WiFi network, and my main "data" use of my telephone is during my daily commute). Sure, I'd probably appreciate the speed increase. But it's not available, so why should I be "bothering" about my data use?
Every carrier in France has an unlimited data plan, mine is priced at €35/month. I could have chosen a slightly cheaper one (€30/month), but I appreciate the extra voice hours. Are they likely to disappear soon? Nope. Orange, Bouygues and SFR (the main 3 networks) have boosted their "unlimited" offers in the past few months, and are looking increasingly likely to start offering unlimited voice plans on their highest price tiers. In six months time, there's a new carrier that's launching (Free), who were the first ISP in France to offer unlimited internet, and they've already announced their objective is to sell a "quadruple play" offer (Cable TV access + Unlimited Internet + Unlimited landline telephone + Unlimited data mobile). Carrier quality? I've had a few data holes, but without moving or waiting more than 5 minutes they're cleared up. They're not very frequent (I haven't had a dropped call, for instance, although I've had 3 or 4 streams that have stopped and needed to be restarted), and they're not seeming to increase in frequency with the multiplication of unlimited data offers.
This plan is a gross attempt to con people into buying into the cheaper plans, and then sucking them dry on "overcharges". 200Mb data? Sure, I'm willing to accept that 60% (or whatever it is) don't use that much. But that's today. What about when iPhone OS4 comes out? Are people going to use their iPhones to "store" media content for the day from their computer/WiFi network and then only use a few tools on the go? Or are people going to adopt new data consumption models, with more streaming and web-based content? I'm pretty sure if you did the same analysis 12 months ago, you'd peg the figures at 150Mb and 1Gb. People are going to get stuck on a two-year plan for less data than they're going to use (only enough for the first few months), and since people will forget to monitor their data use or switch to the higher cap "for that month", they'll be billed overcharges that go well beyond the "saving" that AT&T is offering them now.