Ok guys, I just came back home and had another look at it... and... it's really just nothing... I mean, there's just no way that Apple would replace the entire machine just because of this tiny fault.
I took two more pictures, this time in clear light, just to give you an idea
Here it's totally invisible (I had to use the flash, the picture would have been too grainy otherwise
You can barely see it as a dark "smudge" here
I would feel embarrassed to point this out, so I'm going to keep it. I'm really happy with the machine and I feel that we've already "bonded" anyway

Also, everything else is in perfect condition. No dead pixels etc.
BUT, I will still call Apple, just to see what they have to say. In fact I'll call them right now
EDIT: The guy at Apple said that it is covered by my warranty. It didn't seem to matter how small the damage was. Still, I'm not going to bother with it, I don't want to risk getting a computer with other faults.
Thanks for all your replies.