I got a response from Pebble support on this (via Twitter). It is because of how light interacts with screen/LCD and glue at certain angle and under certain light conditions. Not a defect.
What? It may be known issue, and it may affect most pebbles, but saying this isn't a defect is silly. I do not want this "non-defect" on my $150 watch. If it's not a defect, it's a serious design flaw.
Good idea, we'll look at flipping the colours on notifications. For now, you can definitely use a mostly black watchface to avoid this. It's a factor of different lighting conditions on the lens.
Using a black watch face definitely improved the situation for me and helps lessen the problem, but it does not prevent it. Even using "Text Watch" I get the oily screen effect under many conditions.
I'm not too upset because it's a first-run of an experimental product made by a new and tiny startup. But for the next model/run, if you want to survive in a consumer world with quickly growing competition, you guys may need to get some more industrial design and QA talent. Pickier QA talent.
Not trying to be overly-negative, and I love what you guys have done so far. I'm excited to see where things go. Just my take on it. Keep the cool stuff coming.