Many companies have made diesel/electric hybrid PROTOTYPES.

http://en.wikipedia.org/wiki/Hybrid_vehicle

VW made a prototype diesel-electric hybrid car that achieved 118 mpg US fuel economy (2 liters per 100 km), but has yet to sell a hybrid vehicle.

Dodge had the ESX3 diesel/electric which achieved 72 MPG.

The problem so far: no one can build one CLEAN ENOUGH and CHEAP ENOUGH for consumer viability.

To make them 50-state clean takes a TON of diesel-specific filtering technology and various engine fuel-and-exhaust-processing tricks to destroy or eliminate diesel exhaust particles before they exit the tailpipe. This is very expensive, and has been a major problem.

My prediction (and hope) is that sometime in the next few years, someone besides VW or MB will build and sell a no-compromise 5-passenger diesel/electric hybrid that gets EPA in the 70 MPG range and has about 200 horsepower.

A lot of technology has to get better and cheaper to become viable first though....