From a technical standpoint, there shouldn't be any reason why downloads via wire lines (cable, dsl, fiber) can't stay unlimited. Once you get past the "last mile", equipment to support data connections between the local area and the big internet backbone are getting cheaper and cheaper while their capacity grows better and faster. Not only that, but a couple studies on network traffic have been done and say that data caps do not solve the underlying problem with congestion.
Looking at wireless, there is still some more maturing and need to take place. They are probably 5 years behind where the land line technologies are now. There are a lot more pieces to upgrade and they only have so much spectrum and capacity to work with in a given area. That's why we are seeing carriers moving off of metered minutes and text messages and getting into the data business because they know they can charge people more for it.
Don't get me wrong, I think carriers and providers are going to charge however much they think they can get away with. Looking into the future though, 10 to 15 years out. Your ISP becoming a "dumb pipe" is inevitable.
