Briefly, SDV technology -- which is being aggressively deployed by Time Warner Cable and also by many other large and small systems -- requires two-way communications with the cable company servers to allow the customer to access all of the available channels. Without this capability, those channels on a cable system being managed via SDV would typically be inaccessible to the associated devices.
Since existing third-party CableCARD host devices of types including the new HD TiVo don't currently support the necessary two-way operations, users of these devices (including the new TiVo) could find themselves unable to watch or record channels of interest (the exact set of which will vary from system to system over time).
Looks like, instead of it being an actual interface issue, it's more along the lines of a pissing match between TiVo and the cable companies. Bah and humbug!