My experience sending my D2x and D3 back to Nikon is that they come back looking (and behaving) like new cameras!! They do an outstanding job cleaning sensors and typically will check out other items asked about (like your grease issue).
Since many of you have experienced a grease issue, Nikon needs to know about these observations in order to take corrective action.
My D4 did have some early dust spots that didn't get 'vibrated off' with the internal cleaning routine but, like one other message said, a quick Giotto blow off eliminated them completely.
By the way, if you shoot a photo of a blank, clear image such as a clear blue sky, you can easily identify dust using the magnification button and the LCD right on the camera - great for field issues where your computer isn't handy!