2) As bandwidth increases and the bandwidth necessary to run XYZ application increases steadily over time, when am I assured that prices will change accordingly? A pay per megabyte system on my current DSL line would look significantly different if I was being offered FIOS or a competing fiber service. And 5 years from now, this will look different again. Without competition forcing providers to price down, I could agree to a per-megabyte price that is excellent for YouTube, but is going to blow when I start downloading movies on my 360. Or I could get a plan that's great for downloading movies on my 360 today, and next year is going to seriously hurt when I have a device to download HD quality video. You think the big telco's are complaining now about streaming video, wait until Youtube is regularly serving H264 and beyond.
I have about 100 objections to what the realities of this type of pricing would entail, but those two are good for starters.