I just called nintendo, talked to Mike, he told me that's how it's suppose tobe, it suppose to move alittle.
the ones that are stiff will suffer crack hinges if not used carefully.
I have 5 3ds (got it for the family) and they all move alittle.
if your still are mad at the wobble find alittle thin (I mean real thin) piece of plastic or paper (plastic would be good, if it's too thick use fine grid sand paper to thin it out), cut it into a C-shape like spacer and put it in the little gap and that should fix your wobble problem.
remember when you do this it will be there and to get it out you have to get and thread or thin string and remove it like if your using floss.