What I've been reading today is that the Sky-D meets Euro 6 and US EPA Tier 2 bin 5 without NOx aftertreatment in the exhaust. If so, that is some accomplishment. Perhaps they really mean "no urea" - if so, that's equal to what VW is doing. Other article said 2012 introduction in North America.
But the Sky-G (gasoline direct injection) looks impressive, too. That also uses 14:1 compression (and direct injection and valve timing trickery to make it work). Requires high octane fuel, but the reported fuel consumption is not all that different from the diesel. Mazda is also hinting at the production future of HCCI, and I'd say it would have to be within the production lifetime of this engine, otherwise they wouldn't be talking about the provisions being made for it.
I'm in favor of "whatever works", so if Mazda puts either one into a vehicle that I like ... it'll be on the short list for the next car that I buy.