There are two different things going on.
First, consumers have demanding more and more HD channels. Each HD channel requires between 2MHz and 3 MHz of bandwidth. Meanwhile, old-style analog channels requires 6 MHz of bandwidth. So the most efficient way of satisfying consumer demand for more HD is to trade in each analog channel for two or three HD channels. Fierce competition is what is driving cable and satellite services to go closer and closer to all-digital.
Second, all cable and satellite service providers, that have not already done so, are now encrypting all extra-fee services (such as expanded basic). This ensures that unscrupulous folks don't tap into cable lines to get free cable/satellite service, and ensures that unscrupulous folks don't try to share cable/satellite service service with neighbors. So whatever solution you find must address the need for decryption.
What you need to receive the channels you want using your own equipment depends on which channels you're talking about. If all you want is to receive local over-the-air broadcast channels via cable, generally you only need an unencrypted QAM tuner, which is built into most new televisions these days (and as a result, there are very few stand-along QAM tuners on the market, and they're not cheap).
Note that satellite services aren't so generous - they require their own box even for this level of service.
You indicated, however, that you were interested in cable networks. Therefore, your equipment needs to comply with published standards for hosting decryption devices (known as CableCARD). The desire you have, to use your own equipment instead of leasing additional boxes from the cable company, is held by so few consumers that there isn't a really strong demand for manufacturers to profit from, so therefore there are relatively few devices out there for you to choose from (and they're all DVRs, at this point), and again, they're not cheap. Regardless, with a compliant host device, you'd pay only two or three dollars extra each month for each CableCARD, a fee that is regulated by the federal government, on a nationwide-average-basis.
Note, again, satellite services are not as generous as cable companies: They sought and received waivers of the separable security ban, and so there is no comparable solution for you if you have satellite service.
) The DTAs are a bit limited in capability - they're prohibited by law from being able to support HD - but in the next few months we can expect to see a change that will allow cable companies to use HD-compatible DTAs going forward. That's going to be good news for folks with multiple HDTVs in the house. 
