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.