Email from ISP

On May 17, 2012, we announced the suspension of our 250 GB usage allowance and that we would launch new data usage plans. We will continue to contact the very small number of excessive users about their usage, which can be indicative of security or related issues.

So it sounds like "excessive" is a flexible adjective at the moment. It wouldn't necessarily be accurate to say that there is no cap, because if you go much over 250-300GB they will probably notice and say something if it's a continuous thing.