It's in the picture settings somewhere.
Sky+HD box set to 1080i: HD channels are output at 1080i, plus SD channels are upscaled to 1080i.
Sky+HD box set to Automatic: HD channels are output at 1080i, plus SD channels are left alone & output at 576i (PAL).
http://www.sky.com/portal/site/skyco...icleId=2344410