All nintendo will do is throw it away and send you a replacement. People on here have sent back flashed DS lites with flashme stealth installed and got them changed. Nintendo will have far too many being return to test them. The only way they could test them is to use something like the supercard on them
these are questions no one can answer unless they work in consumer affairs @ Nintendo. Everyone will just feed you what they think or what they've read. Nintendo has made no statements regarding anything like this. But it's pretty straight forward to know that once a ds is flashed, the warranty is thrown out the door....just like every other electronic modified to do things it's not supposed to do.