There's even a nice, well-supported standard for digital transmission on cable in the US: QAM, either QAM-64 or QAM-256. Many, possibly most, digital televisions have built in QAM tuners. There are cheap QAM-to-USB, QAM-to-PCI, and QAM-to-ethernet TV tuners available. Individual programs may be sent in the clear or encrypted.
Cable companies generally send all the local broadcast channels via QAM right now. The more enlightened companies send their basic cable tier unencrypted as well. If there was a good conditional-access-module specification -- the sort of thing CableCard should have been, but is not -- then anyone would be able to call up their cable company, read off the serial number of their device, and get it authorized to decrypt all the channels that they pay for.
Most cablecos are more interested in additional revenue through any possible means than customer satisfaction, though.