I personally don't think it will be completely unlimited, as in, downloading 24/7 at full speed without repercussions. I just don't see it happening IMO. Would be great if they did allow it though, as I would switch come the end of my BT contract, more-so to do with simplifying my bill, as my TV is through Sky.
It will be truly unlimited, just like their ADSL product. It's their USP.