The Nikon MB-D12 Battery Grip for the D800 is now available. Several stores have the grip in stock now for immediate shipping, including Amazon:
By adding the MB-D12, the D800 increases to 6 fps continuous shooting in DX mode. It features magnesium alloy construction and is fully weather sealed like the D800 body. The grip has additional controls for vertical orientation shooting: shutter release button, AF-ON button and a multi-selector dial. The grip includes a battery holder for 8 AA batteries as well as a holder for the Nikon EN-EL15 or EN-EL18 battery packs. The MB-D12 Nikon battery grip is product number 27040. The suggested retail price is $616, and is priced as low as $449 at Adorama and B&H Photo.