Last month, Ars Technica wondered aloud why it is that you can't get a CableCARD device very easily. After all, the FCC mandated that cable companies start supporting "digital cable-ready" television sets way back in July. Despite this, both of the major cable box manufacturers don't sell at retail, cable companies haven't trained their staff on CableCARD and digital TV sets with a CableCARD slot are very, very pricey. The conclusion is that CableCARD's technical limitations mean that cable operators would rather skip it for the Next Big Thing™ to support SDV and 2-way communication right out of the box.
Further confirmation of this comes as TiVo offers up an adapter to allow their CableCARD-ready units to tune in SDV channels, something that they haven't been able to do. Given that most cable providers will be moving to some form of SDV to conserve bandwidth and offer up nearly limitless programming, it seems that CableCARD is a technology headed for the dustbin before it even becomes available.
My personal advice would be to not get too heavily invested in CableCARD unless you get a unit that it specifically upgradeable to support SDV. Even then, it might be worth it to play a game of "wait and see" until the next technology, DCAS, becomes widely available.
There’s always some newer, better technology around the corner. You could keep playing the “wait and see” game forever. I’ve been running my $300 TiVo HD with dual CableCARDs for a few months now and love it. You are right about the lack of training the cable companies provide though. When setting my cards up I called comcast to give them the code from my TiVo as it’s needed to unlock the box on their end. I kept getting told by the tech on the phone that it wasn’t needed and that my cards were active. This despite telling him that they weren’t active and needed my code. I finally had to have an installer come to my place to install the cards. He simply called the office and read them my codes and got it activated in minutes.
The main reason I think it’s worth it to wait for DCAS is extensibility. Yes, CableCARD can support 2-way communications and SDV, but it takes extra hardware and a unit capable of software upgrades to make it happen (which aside from your TiVo isn’t a lot of the devices on the market). Since DCAS is mainly a software solution, it’ll be easy to upgrade and maintain by comparison. It’s the difference between first-gen cable modems that couldn’t go past DOCSIS 1.0 (remember those?) and the newer models that can be flashed to newer DOCSIS standards as needed.