If I had a place to stream it from, I could easily stream an HD movie with my connection. It's not like in the future /less/ people are going to have fast internet!
You are both correct and very blessed to have such a good connection. In my experience, even people with decent internet speed seldom see that speed sustained for the entirety of a large download or stream.
A Blu-Ray disk holds 50GB. At a 10Mb/s connection (fairly common in the US), that would take over 11 hours to download, assuming no interruptions. Even at 100Mb/s it would take about an hour. Assuming you're streaming 2 hours of content you only have a 2:1 ratio of play time to download time, and that's still assuming no service interruptions. Most consumer ISPs promote their max burst rate in their packages, but for larger files kick the actual speeds down.
But again, there's no reason to assume the situation won't improve in the future, even with the exponential growth in number of devices online and increased usage per device.
You could also run a digital "rental" service - the "delivery time" is how long it takes to download when your connection isn't being used. Of course, that would only help people with slow connections rather than those with harsh caps, but it's something perhaps.
Yes the caps would be a pain to get around, but having the BR quality file delivered overnight is a possible solution. Of course when it's movie time you usually select the movie just before watching it (at least we do), so it might be difficult to get someone to accept a 12 hour wait for their 1080p movie.
If you span it right I think you might be able to get away with it - after all, Netflix works and that takes longer than 12 hours! This would also be less effort as you wouldn't have to post discs back.