As an example, look here:
http://www.digital-everywhere.com/en...sid=1190057723
This card has a CI slot built in, but many have additional boards for the CAM interface.
Hauppauge used to have a separate board for their DVB-S cards, but now they just have a USB smart card reader that you can use with most of their digital TV cards.
Using a CI and the appropriate CAM in itself is perfectly acceptable, there are many TV stations around the world that allow you to use your own equipment (or don't even provide any equipment themselves). These services are based on standard encryption systems or will provide the correct CAM to work with their service (TopUP TV for example used to provide a CAM and smart card for use with their channels on Freeview, these went in the CI on set top boxes on in the side of TVs).
However, the two main premium TV services in the UK, Sky and Virgin, both use propitiatory encryption systems and mandate you use their hardware to watch the TV you are paying a subscription for. There are CAMs that reportedly can decrypt Sky, but only with the old blue smart cards, the new white ones don't work with these CAMs. As I said before, I don't know the specifics about Virgin, but I suspect you can get CAMs for whatever they use.