Jay, I don't know why you insist diesels are for larger vehicles only. I'm sure one could engineer a diesel motorcycle if needed. The article this is based on is about the new clean diesel. This isn't the same as the diesels in current vehicles. It has a filter up front that increases the amount of oxygen available to the engine to burn the deisel fuel more efficiently and therefor cleaner and even more efficient than existing deisels. It includes an additional filter on the exhaust to remove more particles so in effect the engine cleans the air. I feel I have to repeat this because so much of this discussion seems to have missed the point. A more efficient engine that burns less fuel and therefor pollutes less is a good thing.
The article goes on to point out the problems. The biggest one I see is that there is only a certain percentage of diesel in a barrel of oil. It is much less that gasoline. This is why diesel fuel is already more expensive than gasoline. Having more diesel vehicles (efficient or not) on the road will only make the fuel more expensive. Still more options is better than less