What is the role of (bio)diesel in a green transportation revolution?  i think this hybrid has gone wrong, VW ought to go with the parallel hybrid (electric plugin rear axle drive, front wheel drive ICE) Audi design hybrid for now.  Then move onto this...

http://gristmill.grist.org/story/2008/3/4/10116/33017#8

The future of diesel.
That's the interesting topic.  I think it is in backup generator power for a plugin hybrid.

If 6 kwh is used in an hour to go 55 miles.  A 10kw diesel generator will do the backup job nicely.

With a conventional diesel generator a turbine generator can be added to improve efficiency and get many time the stadard internal combustion driven vehicle efficiency.  Turbocharging is a fairly normal option on cars now, so it would not be a stretch to mass produce this feature.

I think that solid oxide fuel cell/turbines are the next step, but it doesn't matter.  Diesels with the turbo efficiency booster are good enough.  And both of these options can be adapted to use biogas very easily.

Making your vehicle backup power, backup for you home or local grid when hooked up to biogas.  Compressed biogas to run plugin hybrid farm equipment is also an option.