While I hope you are right and they can offer HD streaming as a low fixed price option I am not holding my breath.
Part of what makes disk by mail work is that there is a limit to how many disks you can get during a month. If it ships today, you get it tomorrow and watch it and then ship it back the next day that's a 4-5 day cycle.
If you stream a movie at 6:00 in the evening when you are done you can stream another one at 9:00 or 10:00 if you want. And on the weekends or weekdays spent at home the number of movies that can be streamed goes through the roof.
So, unless the system enforces download limits streaming can easily deliver 5-10 times as many movies as disk by mail. Assuming that the studios want to make money on this I don't see how streaming can be as cheap as you hope it will be.
Most of the existing services ( AppleTV, Vudu etc ) charge $5-$7 per HD movie. I don't doubt that NetFlix may be able to beat this but I don't know by how much.
Just my 0.02 worth ... Tim