I'm sure that none of you really think that a cable modem is a modem

Correct, we know it is

since it doesn't do AD/DA conversion but we all understand that it's simply a device used to connect a customer to a provider.

No, it's a device that converts your data into an analogue signal (modem) suitable for carriage on broadband plant (cable)

After all "cable modem" is nothing more than a marketing term so that customers have a rough understanding of what the device does (not that they always understand that either).

No it's an actual device that performs the function correctly denoted by it's name. Broadband is the marketing term.

The question then remains: What (in your opinion) constitutes broadband according to the services that have been promised to consumers but not yet delivered?

The rates obtained from current day broadband systems (e.g. cable modems & *dsl) are normally what is being referred to by the contraction "broadband" brandon