The display on my brand new 2015 13" rMBP cracked in a very similar way yesterday, ONE DAY after I got it. Why they don't use overlay glass like they have in the past in previous models is beyond me. We're basically looking at a screen with the fragility of an iPhone's - except it's several times larger. I've owned 4 or 5 previous Apple laptops and NONE of them have been this prone to damage.
Anyway, I spent about an hour at the Genius Bar and was told that I'd be looking at a $450 display replacement repair. That wasn't exactly the kind of news I wanted to hear after I had just spent $1,700 on a new laptop I only got one usable day out of. But I did get a manager involved near the end and they agreed to cover half the repair cost. So for that, I'm grateful. But OP, YMMV. I'm not totally sure whether they're replacing just the display or the entire assembly on mine. I was so upset I didn't think to ask. So, it's best to make an appointment and see what they say.
I've also been reading how micro scratches caused by dust/particles on the keyboard can also damage a Retina display. I guess a screen protector is going on my rMBP the second I get it back from repair. I never thought I'd need one for a laptop...