Hi everyone,
I also had this issue with my late 2008 13" unibody macbook last summer. I took it in since it was still under warranty and they replaced the display. This was near the middle of the display, but not over the apple logo area. The shape was two lines kind of in the shape of the letter T. Here is their exact description on my invoice:
Confirmed. Artifact only shows on black background of LCD screen tester. Entire artifact is approximately four inches in length and is visible starting approximately 1.5 inches to left of centre to approximately 2.5 inches to right of centre. Height is approximately 0.5 inches above centre. Note that artifact is made up of a series of smaller artifacts. Appears as though a soft, blunt instrument may have ben run lightly across LCD surface prior to display final assembly.
The weird thing is, I just noticed about a month ago that my new display has the same thing again! This time it's not at all near the centre. It's on the left side of the screen and near the vertical middle. I am going to take it in again even though my apple warranty is expired since the parts replaced have an extended 1 year warranty. Another thing I noticed is that looking REALLY close you can see black lines forming a grid over the artifact. Almost like black lines around the pixels.
If you have this on your macbook and are under warranty, take it in! They should replace it. However, if you no longer have a warranty, you're better off learning to live with it since in order to fix this they replace the entire display (screen, cover, iSight camera) which will set you back about $700 for a 13".
HTH.