If I'm not mistaken, MythTV already supports a multitude of cable cards...
Do you have one now? What kind/model?
Chances are it's supported by myth, and you should be able to find drivers for it..
mythtv supports a HUGE library of tuner cards, ATSC, DTV, Cable, DVB-T, DVB-S, DVB-A, etc..
Hope this helps,
(I'm just browsing while linuxmce 1.1 downloads
Good Luck, let us know if you find support for your card...