Modern smart phones enable MUCH greater data use and the carriers are charging for it. If they offered the more capable phones at the same rate as the less capable phones then the folks with the less capable phones would be, by comparison, taking it up the *ss. The carriers spend money to install and build out a network that they'll need to replace in 3-5 years and you want that for free? Build your own network and we'll see how you manage to stay in business when demand increases MUCH faster than income...
Before smart phones the non voice data use was less than a few MB/month. The early era of smart phones saw the data use increase to a few 10's of MB/month. New smart phones like the Evo are eating 100's of MB/month with many well into the GB/month range. There's even one guy here that brags about eating over 50GB/month.
When the new phones eat 10X-1000X the data of prior phones you have to expect the carriers to charge for it.
Brian