(NOTE: Comcast Call center rep on forums between calls/at home on forum. Not representing comcast in forums)
As I understand it, we have to send a "bootfile" to your modem that tells it how to work, we send it via a broadcast and only the modem with the right mac address will accept it.
That Bootfile will tell your modem what speeds its allowed and other specifics like ip's for the dhcp server and such.
I don't know how the bandwidth meter will be setup but hope people will be calm/civil about things till it becomes a issue that affects more people then are being affected now. (IE when 250 starts to feel like a small # in terms of bandwidth)
Comcast assigns IPs via DHCP, and they control the DHCP leases by MAC address of the modem, router, or gateway device. (You find this out if you try to replace the device they know about because until they add it on, it won't work.)
So they already have your IP address in a database somewhere with the other account information. It isn't too much further to add in a bandwidth meter to it.
And since you can view your account information online, adding a line to show bandwidth usage is not at all difficult.
I think it was at LifeHacker.com that a guy representing that he was a tech guy @ Comcast who said a bandwidth meter was in the works.