Further to all the discussion in this thread about HD, it would occur to me that what would be really cool is to see an 'Also in HD' overlay on an SD channel when the programme is being simulcast in HD. Hitting that colour (hey, use blue) and it'll pop over to the BBC HD channel. Neat.
I don't think Sky allow you to switch TV channels using MHEG, which is a shame, so I guess I'll never get to see that on my box... J
