Effective June 1, 2006, refiners and importers nationwide are now required to ensure that at least 80 percent of the volume of the highway diesel fuel they produce or import is ULSD-compliant.

The Audi 3.0 TDI engine delivers fuel economy levels up to 14.5 l/100km with a CO2 mass emission of only 282 g/km.

Starting in 2008 Audi will bring 3.0 TDI to Q7 and new A4, it is also expecting to put it into new A5 (at least I hope they do).

A hybrid TDI could probably give you between 70-90 mpg and you may have to pay 5000$ more for that technology.

Let's do some math.
I have a 2000 Pontiac Grand Prix and get average 21mpg.
Based on http://www.fueleconomy.gov/feg/noframes/15949.shtml
it costs me 2142$ to drive 15K miles a year. (I live in CA so its more like 2356$ since I pay about 3.30$ for each gallon of gas)

If I had hybrid TDI, and pay same for gas or bit more lets say 3.45$ for diesel. Let's say I will take the average of hybrid TDI so fuel consumption would be 80 mpg. My yearly cost of driving 15K miles a year would be 646.87$. I would save 1709.16$ a year. So within 3 years I made my money back.

My next purchase will be a car with TDI. Audi just introduced a pototype of A3 in Europe with Hybrid TDI technology.