Puncher:
My understanding was to record two programmes simultaneously, while watching neither and be able to watch terrestrial TV (or other non-Sky+ TV source) at the same time.
Hmmm. Perhaps you can do this if you had Sky+ and the built-in DTV module and you watched that whilst Sky records. I don't know, I've never tried it (although I will do).
I don't know if the Sky HD box is capable of recording two HD programmes to the drive at the same time, due to the amount of data it would have to write to the drive, simultaneously.