It's just coming in the CX-5 first, not the 6 sedan.
The diesel in question is Mazda's Skyactiv-D 2.2-liter four, which makes 173 horsepower and 310 pound-feet of torque in other models. Putting it in a small crossover makes some sense, especially considering the recent announcement of a diesel Chevy Equinox.
It's good news for choice and another sign that the diesel scandal brewhaha's effect on diesel decisions is blowing over. If it works in the CX-5, we may see this option spread throughout the Mazda lineup. Now if only someone could make a case for that rotary...