Just tested the unpaired card issue on a Thomson HD receiver and a card paired to a Pace Sky+.
If I put the card into the Thomson, to which it is not paired, I can see that:
Recordings already made on BBC HD and FTA channels work normally, playback is possible.
New recordings can be made and viewed on free to air channels only, including BBC HD.
A recording can be made on an encrypted channels such as Sky One, Channel Four and Five, but cannot be played, showing the message 'recording interrupted'.
Existing recordings on the packages that the Sky+ card is enabled for cannot be viewed either, that is those made with the box's paired card.
In short, when swapping a card between Sky+ boxes, recording will only work normally on FTA channels.