They do that now. They tell you what you can or can't do with your smart phone wither you buy it full price or not. You can't buy an iPhone from AT&T, G1 from TMobile or Palm Pre from Sprint without a data plan.
That only applies if you use it on their network. ATT can't tell you what plan you need if you cancel your service with them and try to use the iPhone on T-Mobile... But I see what you are saying. -- "This is a bus. You know how big a bus is?"
