I am coming to the conclusion that I better get my IPhone 4 now, and unlimited data, even if I will NEVER come close to using even 2G per month. I just pisses me off that they could not come up with something better, especially for folks who have family plans with multiple lines. I really hate giving them $60/mo for two line for two IPhones. But, like you said, at least when the 7 comes out I will still be unlimited. I am actually even thinking that maybe I will turn on the unlimited data, and then wait for the 5 to come out since I am past contract on the LG feature phone.
I did read one rumor that said data would be sharable in a family plan. Probably why the unlimited option was removed. Can't have more than 1 user sharing an unlimited plan for $30. They would be hurting. But if people pay $50 for 5GB, that's 2.5GB each and cheaper than paying $30 x 2phones.
In my case, my wife is not interested in a smartphone, so I'm just paying for one line of data. Also get her state employee discount of 20% so that's a great deal.