Anybody remember the Romanian ARO from about 1997-99? Same basic story.
Breaking into the US auto market is not an easy thing to do. Sooner or later one of the current companies will figure out a way to bring a small diesel into the US market, I think the hard part is going to be making a vehicle that is (a) powerful enough to please US consumers, (b) economical enough to justify the additional cost of a diesel and (c) able to pass US emissions (particularly in California.)
I think it's one of those "pick any two" type of situations: Make a vehicle that is powerful and passes emissions, and its fuel economy will be just as bad (or worse) than a gas vehicle (that of course burns cheaper fuel since diesel is anywhere from $0.20 - $0.80 more expensive in most US markets); Make it economical and emissions-compliant and it's slower than a VW bus, which makes it a non-starter with most US consumers; Make it powerful and economical and it won't pass emissions which means it can't be sold in the US.
As for who will bring a small diesel to the US, my money's on Nissan. Nissan seems to be 'hungry' in a way that Toyota and Honda are not. I guess it's like the old Avis rent-a-car "we're no. 2 so we try harder" mentality or something but Nissan does seem to be willing to take risks that other automakers won't.