I think we are moving to a digital era where more and more mediums are becoming digitalized (digital game downloads, hd movies, etc). I think when the digital market matures your 250GB cap won't look so as big as it does for you today.
While I was working for a cable modem provider in the early 2000s, I learned several things. One thing that really stood out, was that providers do not care about innovation, they do not care about the future, all most providers care about is how much money they are going to make today or tomorrow.
Take SBC for example in the late 1990s. While time warner was rebuilding their cable plants, sbc was sitting on their hands. It was like sbc did not care that time warner was scooping up customers by the thousands. In some of the cable plants I worked in, dsl was 4 - 5 years behind cable modem.
While the 250 gig cap might be fine now, in the near future its probably going to run out rather fast. When that time comes, providers will probably offer a tiered payment plan. I dont think providers are really looking towards the future, except how much money they can squeeze out of people.
In the long run, I look for the USA to fall further and further behind other countries in the high speed market. with greed becoming the final factor on whether to upgrade the lines or not.