Nikon USA/Nikon EU/Nikon Japan issued a service advisory for thermal problem (white dots/bright spots) during long exposures with the D810 camera. Back in 2010 the D7000 had a similar issue and I am surprised that Nikon has another QA problem after the D600 spots. At least this time they offered a solution just few weeks after the camera was released.
This internal memo indicates that the problem will be fixed with a combination of a firmware update and "pixel defect compensation":
We have received a few reports from some users of the Nikon D810 digital SLR camera indicating that noise (bright spots) are sometimes noticeable in long exposures, and in some images captured at an Image area setting of 1.2× (30×20).
After looking into the matter, we have determined that some noise (bright spots) may on occasion be noticeable when shooting long exposures, and in images captured at an Image area setting of 1.2× (30×20).
Nikon service centers will service these cameras that have already been purchased as needed free of charge to the customer. We sincerely apologize for any inconvenience this issue may have caused.
Identifying affected products
To check whether or not your camera is one of those affected by this issue, please click the Affected Product Serial Numbers link below and enter your D810’s serial number as instructed. Your camera’s serial number will be checked against those of affected products. If your camera is one of those affected, you will be forwarded to additional instructions. If your camera is not one of those affected, you may continue using your camera without concern for this issue.