I've read online (DSL reports) that Metrocast uses the data caps as a "line in the sand" for those that outrageously go over the caps, and that they ignore users who go slightly over. You might not have been aware, but there have been caps in place for some time. For example, the old cap on the highest service level was 350GB per month, while the new cap for that same service level is 1TB.
Since the caps are pretty generous now, I wouldn't be surprised to see them give out warnings for those that go way over, but haven't read anything yet about charging for going over. If you have a large household where everyone is streaming video every day, 1TB might no be enough for your needs but for most people, 1 TB should be fine. 1TB is about 33GB of data per day.
For a 2 hour Netflix movie stream of 3-4GB, you could watch 8-9 movies per day, 7 days a week, and still not go over the new cap. But larger households with multiple users may have more difficulty staying under 1TB.