Why would it need to be done at the backbone?
Isn't it possible to have a statistically valid sampling of packets even at the highest speeds? You'd know better than I would. I hope the answer is yes, because that is valuable data even if not used for billing. -- Shields, CrossLink.
And when someone argues that your data and/or collection methods are flawed and refuses to pay, what do you do? The circuit switch world that people are comparing this to keep full records at all network ingress/egress points, which between ISPs are often their busiest boxes. AT&T does not trust PacBell to just send them the money, both sides account each transaction. What is valuable for engineering and what is required for accounting and billing are way different, one has to be able to stand up in court. The other point of unmetered usage is a direct motive to keep the growth of use as high as possible. Some people believe that the "meteoric rise in Internet usage" is an important part of marketing plans and thus things that reduce it are bad. I have certainly used arguments like this in the past to argue against metered networking in corporations. You may kill far more than wasteful use if you discourage people from using the Internet whenever they want to, which metered billing would. jerry