Apple refusing to fix. Known issue since 2008: https://discussions.apple.com/thread...art=0&tstart=0
Example photo: http://i44.tinypic.com/35lgupv.jpg
Applecare first confirmed they would replace the unit, just take it into the store for a straight swap. However, the Genius staff have never seen it before and so refused to fix.
This is reminiscent of when the white plastic macbooks developed cracks and they refused to be replaced/repaired by Apple.