At some point in the past, cable boxes were simple things that just changed channel, now they have a whole host of different states like on demand that can really confuse mythtv.

I've set up channel 109 (which is On Demand Preview on the Virgin box) to put the box into on-demand mode, giving mythtv a way of knowing the cable box is doing something other than displaying one of the conventional channels. It relies on users using channel 109 to switch to On-Demand mode, so isn't perfect, but it seems to work.