I am a huge fan of diesels, enjoy the way the make their power and run on the road, so more than anything I wish Ford had a diesel offering in the Fiesta or Focus. Unfortunately, as much as I hope it changes in the future, I understand the logic of not expanding the diesel passenger car market to become anywhere near a significant proportion as it would lead to a rise in diesel prices which would in turn raise prices on just about every item in every store. VW, MB, and BMW have their diesels, but they combine to account for less than 3% of the market while domestic pickups with diesels often are used in the process of getting goods to market. Diesel seems to be the US's commercial fuel and domestic manufacturers nor the government are looking to change that. Possibly should algae-based biodiesel or another biodiesel ever become more viable in terms of supply this could allow for a policy shift to increase passenger diesel availability.
I think the best business decision for Ford in the US, if they wanted to explore a diesel car, would be in the upcoming Focus line that is likely to generate the Focus, an Escape/Kuga, and possibly a C-Max type MPV. That would be in addition to the loosely Focus-based Transit Connect. Only with the ability to install the same engine in multiple models would the EPA certification and expense be potentially recoverable. Use in the Escape/Kuga would likely dent the demand for the hybrid version which would be a contraindication, and Ford has already announced an electric Focus and Transit Connect, so I'm not seeing a diesel Ford car in the near to mid-term. That leaves Ford's 4.4L V8 diesel that is on indefinite hold for the F-150 and the larger Super Duty diesel as the only diesels on the horizon in my opinion.
Which brings me to my justification for going on and on about the broader diesel picture in a Fiesta discussion. I will be in the market for a new vehicle in 2011 provided a product exists that sufficiently interests me. I am hopeful the Fiesta will be just the car and I would be willing to pay $2-3k extra for a diesel engine. At the same time, if the 4.4L V8 diesel is available in an F-150 and it returns respectable fuel mileage I'd be interested in going for a relatively basic truck in the $25k range. Personally, I would have preferred a 6-cyl diesel making 30mpg feasible on the highway, so I'm thinking the V8 may not be my cup of tea even if it is offered.
The take rate for a diesel Fiesta would not be high, probably 15% tops given the premium it would require, but I would certainly be in that group.