It's amusing, and I think it's coming about because there is a trend towards small-capacity diesels in some up-market vehicles. BMW are introducing a 0.8L TD. Audi have TD across a number of their range. Even the garden variety manufacturers are getting into it, I'm seeing a number of diesel Holden Cruzes and Captivas kicking about.
"Premium" diesel is a way to appeal to the snobby bastards that don't need to indicate to turn left from the right lane on a freeway, or double-park across your driveway because they want to have a half-hour conversation on the phone to a sex line and can't use their steering wheel with their knees while both hands are busy like that.
Caltex in NSW have several outlets with signs up out the front showing "Truck Diesel to the left, 144.9cpl and Premium Diesel to the right, 146.9cpl". Next time I see one and I remember, I'll take a picture of it.
Personally I don't see why they have to introduce more reasons for personal self-satisfaction. Penthouse ought to be enough already.