i dont think this is something to worry about unless it efects your game play. i have that "crack" and it dose not bother me. i have a simalar problem with my 3d slider. it semes to get stuck on itself. i think there is nothing to worry about but if it affects game play or really bothers you you can set up a repair at this number 1-800-255-3700.
ps i have herd that nintendo might not retern, might loose, your styules or sd card so unless they tell you to send them to mack shure to take them out.
I just bought my 3ds and within 4 hrs of using it the circle pad gets hung up on the edges arround it. It really effects gameplay when suddenly you can't move your character. I think I will return mine. It's a shame as the rest of the experience with it has been fun.
owch. i think that is a problem with your 3ds. try to return it and get a nuther one