For all those that don't know, the original crack is caused by opening the DS to the second 'click', so it lies completely flat. If you only open the DS to the first click, you shouldn't have any problem. My wife's wasn't cracked for months and then she accidentally opened it all the way. She closed it, and sure enough, there was the crack.
A stupid design flaw, very surprising that it made it past Nintendo's usually very stringent testing. We're pretty rough with ours (played Planet Puzzle League against each other for nearly two hours in line for Finding Nemo Submarine Ride at Disneyland yesterday) and the crack hasn't caused any new problems. Still, an annoying defect on an otherwise nearly perfect handheld.