I was wondering about this also, the box it self has been dismantled and chipped and is well documented over at t-hack but I dont think anyoe has looked at the encryption before no one was bothered but now they are going to offer sky sports I would like to look into it but they havnt announced how they are going to deliver the picture , I cant see it being broadcast via terresital more likely via iptv
BT vision boxes dont use cards so dunno why yours has one inside.
BT use a cardless certificate based system which, to all intents and purposes, is uncrackable.
The boxes themselves can, however, be played with to allow them to be used as pvr boxes on freeview if you have sufficient technical knowledge. Afaik, there is no simple "readme" on doing this - you actually need to know how to get your hands dirty and do some real coding !