While most people see the gasoline-electric hybrid car naturally evolving towards the plug-in concept, it's not so clear what will happen with European diesel powered cars. While the diesel is clearly more efficient than the spark-ignition engine it is also, and has always been, more expensive to produce. Generally speaking the hybrid electric vehicle is the most efficient in city-type driving while the diesel is superior for highway speeds and distances. Marrying the diesel to the hybrid concept would seem to give the best of both worlds it would have a major impact on the sticker price. Current fuel prices are not high enough in North America to justify either technology with the exception of high mileage users like taxis.
The plug-in vehicle concept enables a car to travel a typically commuting range on electric power alone while retaining a gasoline engine for longer trips. This maximizes flexibility while reducing the need for a huge and expensive battery pack. The introduction of the plug-in concept reduces the significance of the internal combustion engine to that of an on-board generator however. This begs the question, will there ever be value in trading a gasoline engine for a diesel model on a plug-in hybrid? Which improves fuel economy more, swapping the gasoline generator for a diesel or adding additional battery capacity? E.g. for the price of changing to diesel you can add 20 km of all-electric range through batteries.
It's not practical to answer this question without data on driver's habits. We would need to know the actual distribution of trips in terms of distance and type (city or highway) at a minimum to evaluate which gains the most: extra battery capacity for more all-electric kilometers or improving the efficiency of the on-board generator. My suspicion is that the diesel does not come out ahead for the majority of users unless they have a very long commute, such as someone living in exurbia.
So where does this leave Europe? In a sad way, I think by trying to be progressive they have taken the wrong path and are now tied to the diesel concept. This may be another case of governments picking the loser. Government is historically inept at choosing the right technological solution and should whenever possible simply set criteria on environmental/energy issues and avoid endorsing particular solutions.