The infrared cut filter has been modified for the hydrogen alpha wavelength in order to capture diffuse nebulae in the night sky. It provides four times greater sensitivity of the 656nm wavelength. Additionally, it adds a new long exposure manual mode with shutter speeds possible from 4, 5, 8, 10, 15, 20, 30, 60, 120, 180, 240, 300, 600 or 900 seconds (15 minutes).
The D810A offers a sensitivity range of ISO 200-12,800 (expandable to Hi-2 51,200). It also has a new Virtual Exposure Preview Mode, “which displays an estimated preview image and is available when shooting at shutter speeds longer than 30 seconds when in Live View. The brightened preview image represents a 30 second exposure, simplifying focusing and composition.”
Virtually all of the other Nikon D810 settings are available on the D810A; however, due to the modifications, the D810A is not recommended for general photography.
The Nikon D810A should be available in May 2015. Pricing has not yet been announced.
Update: It looks like B&H has listed the price at $3796.95. Check it out here at B&H Photo.