Yes, we have gas differences by region here in Cali.

They've tried various forms of additives that are supposed to cut nitrides of oxygen (NOx). Since more driving is done in the summer months, they add more of these compunds to the gasoline than in winter. One of the more recent additives was Methylterylbutylethylene -- or MTBE. A few months after it's use was approved by CARB, it started showing up in groundwater and runoff. So that was the end of that expiriment. I have no idea what they are using to replace it.

Emissions-wise, I think the next big improvement will come in catalytic converter technology. The new challenge will probably be figuring out how to light them off faster without having to integrate them into the exhaust header.