The US has new regulations that make new diesels excessively difficult to get certified. Both of the existing processes are somewhat cumbersome and expensive, and neither will fit easily into a smart car.
A similar problem is about to exist in Europe, which until now has been diesel-happy. Their new regulations kick in over the next few years and will bring them up to (and eventually surpass) the US regulations. As I recall, even the current US-approved Mercedes Bluetec system won't meet the upcoming European regulations, so everyone is scrambling to figure out something that will work.
The Canadian 450 diesels don't pass even the old US diesel regulations, which is why they cannot be imported into the US.
The smart microhybrid only improves mileage by 10-15%. The 89 mpg hybrid that I suspect bentoutofshape is talking about is the VW Golf diesel hybrid concept that was shown at the Geneva auto show. Fantastic application of technology, but VW hasn't committed to making it at all yet, let alone to importing it for the US.