There really isn't anything that says the warranty is void when you open the DS or get one "used". I think they find out by seeing any wear and tear on the inside. If they see any knife marks on the rubbers, then the company will suspect you of opening the top part. The only thing you can do to your ds that will NOT void the warranty is putting decals on it. Anything else that basically screws with it cosmetically or anything in terms of function, then it will void the warranty, so you might as well open it because most problems get caused by going deep inside the DS, like the ribbon cables, or a fuse or so.
