For several years, this system was heavily restricted with the unspoken intent to prevent manufacturers from producing 3rd party devices capable of interfacing with digital cable systems. There were only a handful of devices ever authorized, including one tuner card, several TVs, and higher end TiVos. In late 2009, the FCC declared CableCard a failure, and under threat of having to replace all their existing hardware under a potentially less favorable system, the restrictions were lightened, resulting in three tuners now available for use with MythTV: the HDHomeRun Prime, the DCR-2650, and the InfiniTV 4.
Copy Control Information (CCI)
While MythTV can record from the above devices, the CableCard system is still a DRM implementation, and since MythTV cannot support DRM, it is only allowed to access those shows the tuner deems as DRM-free. What content this actually ends up being depends on your cable provider. Ronald Frazier has started collecting information on what areas should have what channels available to record, and has tabulated it here.