I have a similar crack, just a bit further towards the keyboard and about the same length. I assume it comes from the raised surfaces at the top of the lid.
I've never asked Apple to take a look though, I'll just live with it.
Yeh I wouldn't mind too much either but I want to sell it and it'll be harder to sell now with this fault
Yes afaik it's a known issue and they should replace the casing for you.