Sprints WiMax 4G network is probably the closest thing to what the future of data plans would be. They have the ability to just pay a daily rate on days you use the service. The problem of course is that it is only deployed in 2 or 3 cities right now.
What it comes down to at the end of the day is just a lack of bandwidth on the cellular networks currently. I think in the future with 4G networks you might start to see, as you already are with Sprint the ability to use a cellular network as not only a mobile network though one that is open and fast enough to be used as your home ISP.
With that said, I think it is rather dumb that Verizon and ATT have not yet figured out how to offer a deal to consumers to group together DSL/FIOS and mobile broadband.