This just in - Nikon released a third service advisory on the D750 shading issue. Now cameras produced between December 2014 and June 2015 are also being recalled (the previous timeframe for impacted models was October - November, 2014):
In July of 2015, we announced that the shutter in some Nikon D750 digital SLR cameras manufactured in October and November of 2014 may not function normally, sometimes resulting in shading of a portion of images. Since that time, we have learned that the same issue may also affect D750 cameras manufactured from December, 2014 through June, 2015.
For those who have already purchased a D750 and would like to have their camera serviced for this issue, Nikon service centers will service cameras as indicated below. We sincerely apologize for any inconvenience this issue may have caused.
Identifying affected D750 cameras
To check whether or not your camera may be one of those affected by this issue, click the Check Your Serial Number link and enter your D750's serial number as instructed. . If your D750 camera is one of those that may be affected, instructions for obtaining service will be displayed. If your D750 camera is not one of those to which this issue may apply, rest assured that service to your D750 camera as to this issue is not necessary and you may continue using your D750 camera without concern for this issue.
If your D750 camera may be affected, as confirmed with the serial number check above, you will be provided with the necessary information to obtain service for this issue free of charge. Once your D750 camera is received by Nikon, your D750 camera’s shutter will be examined and replaced, and your D750 camera returned to you free of charge, even if your D750 camera warranty has expired.
Via Nikon USA