Quick follow-up from Bob: I noticed in most/all of your random photos in the galleries that I viewed, using normal to telephoto focal lengths your corners are present.
Bob
07-Jan-2010 06:16
RE your corner vignetting in the tropical lagoon photo: You have a zoom lens, and the pic seems to have been taken at a very wide focal length. If the lens does not fit the sensor sometimes the image at the back of the lens is larger than the sensor at the widest focal length. You can see it in the viewfinder. Try backing off from 18mm to 25mm, or so. See if the corners re-appear.
