I have a Radeon 8500 and see a very similar effect. On my ATI, coastlines often appear as whitish outlines that look too bright. But the weirdest effect is: I can even see them THROUGH MOUNTAINS!! When flying through hills close to coast I can often see the coastline (just the bright outline) even though it should be masked by the hill. I can't remember seeing it lately, so it's probably driver-dependent.