$30 is the same as other smart phones, so I say that's fine. Though, unlike other smart phones, with the iPhone you are required to pick a data plan with the 2 year contract, and it's unknown if you can remove it at will with the 3G iPhone. So you'd think, for making it required, they could have added some kind of discount, like a $25 plan, or tethering as someone suggested.
And $30 doesn't seem that bad for a device that can transfer so much data. I think the only ATT data option for my non-smart phone is more, a Sony Ericcson W810i walkman, is a whopping $35, but it's some bundle of media features that includes text I think, but again, not other data option. Right now, a little news browsing has done 0.23 megs of data, which without a plan translates to $2.30.
I don't use data for a Ericcson much, and a no data plan works the best, but I hate how ATT only offers a no data plan at $10 per megabyte, or a whopping $35 a month, nearly doubling your bill for a reg phone that doesn't do much data,. And the phone has a large button that takes you to the media net front page which gets hit all the time in my pocket. Making it almost unavoidable to end up with a $1 data charge a month without even using it. One time I pulled the phone out and found it 6 pages into the net. My phone has no games because just downloading it to the phone would cost more than the game itself, so I only check headline news now and then.