Bob, Looks like you had a dry fiber on your sensor. I also have my D300's cleaner set to clean when turning the camera on and off, and this often takes care of the dry bits. If you get an oily one, though, it could be time for a wet cleaning.
If I were you, I would probably use a blower brush like a Rocket Blower, (no bristles) and blow the camera out with the lens off--it's important to blow at an angle to the sensor on both sides. That way you can get rid of that little stowaway for good!