Or could it be they are rolling out dynamic ad breaks into live channels (with ad breaks stored on the Sky+ HD being triggered to drop into slots?)
Are they not doing this already? Can't say I watch much of their channels live, but a friend who works there was talking about it a few years ago.
Already doing it and have done for a while. It's called AdSmart and its only on the Sky channels and a select few others.
The neat thing for advertisers if one records a programme that has an AdSmart slot in it and you play it back months/years later, the ad in question is a current one, not one of the ones that was around at the time of the recording.
There's no mention of it on the site but I dare say if the hard drive packs up or is non existant in the Sky+ box, AdSmart can't work so you'll just get a generic ad instead.