I heard an interview with the big cheese from Sky who said they know which channel all their decoders are tuned to. Also, they know if you have MYSky'ed a program and when you subsequently view it. Interestingly, if you record a game/program and watch it within a week of the original air date it gets counted in the viewership figures.
I wouldn't be surprised if they can tell the difference between a decoder being on a particular channel and the tv is on or off. I'm pretty sure if you try to change the channel on Sky when the TV is off nothing happens. Therefore the decoder knows when the tv is on/off, therefore Sky know when the tv is on/off.
You can absolutely change the channel with the TV off, just not with the decoder off.
Really? I thought you couldn't. Right that's it, I'm taking the afternoon off work to go home and double check!