No, I think its going to be a service book that is sent down by the provider. I also have a small sneaky feeling that it may be an extra few bucks a month for this said feature.
I don't think you are wrong about modem support on the new devices costing a bit more, its another reason to think that it won't be available on existing hardware. I doubt it will be due to a service book though since they have other means of controlling access as bwc users in the uk among others have already seen.
Since its new hardware they'll be able to sign you up to a new tariff, they won't want people on an 'unlimited' tariff having the ability to use at least 10x as much traffic without paying accordingly.