Since it's not a real thing yet, there's no way to know for sure. I seriously doubt that Apple would drastically reduce the battery life on the MBA line, but by converting the display to retina, the existing battery configuration may be affected. I would guess that Apple is working to implement more advanced battery technology before going retina on the MBA.
MacBook Pro still has more power. But if Apple did release an Air with a retina display, I'm sure retina MacBook sales would become almost non-existent.