What happened in Canada is now happening to the United States and I'd wager the same results will come in.
About a year ago, Bell introduced a 5 GB monthly cap on their DSL. Additional capacity costed C$0.80 per GB up to C$30. So in essence, people that downloaded a lot paid C$69 a month. This angered a lot of people and in the months after that there was a great influx from Bell to Rogers and Cogeco, the two main cable companies. Some of the other DSL companies got some customers, but for most part people had the "DSL sucks" in their head and wanted a different technology.
Bell Canada must have realized that **** is flying because they are loosing all their customers. I think it severely hurt them. So, in July they announced that they are removing the cap completely, and that they are increasing the speed from 1 Mbps to 1.5 Mbps. Quick change of tune, eh?
So what happened elsewhere in Canada with the other companies?
Well, Rogers, the biggest cable company, considered caps for a while but dismissed the idea after seeing Bell loosing half their customers. Rogers now offers 1.5 Mbps / 384 Kbps as a standard package with no cap.
Cogeco, my cable company, has always had a 30 GB soft cap, but they have never enforced it. According to them, it is there for legal reasons. I've downloaded more than 100 GB some months and they didn't do a thing. I don't think they are going to change their tune either. They are trying VERY VERY hard to get customers. Their standard package is 3 Mbps / 400 Kbps, and their professional package is 6 Mbps / 640 Kbps. They are DOUBLE the speed of Rogers and Bell's DSL and they use that to get more customers.
Shaw's, another cable company, standard package is 8 Mbps. They have no caps and while they considered it decided to put the idea away.
The other DSL providers such as AT&T, Primus, and so on, used to have caps but are now cap free.
This capping thing is really a money making scheme. Cable and DSL companies are getting greedy and wants to get every possible penny out of you. This costed Bell Canada dearly, and I'd wager the more users get grumpy by the cap, the more users will leave.
In Comcast's case, a 2 GB per day cap isn't too bad. 60 GB is a lot per month.
In the end, the users will be either happy or unhappy, and unhappy people tend to take their business to the competitors. This capping thing will not last. Canadian companies tried it and it failed miserably, and the Yanks tend to be more vocal about these things. It just won't last.