On the 747, at least, it depends whether the inner or the outer panel that suffered the crack. If it is the outer panel that cracked, it is not so much a problem as if the inner panel cracked. The checklist calls for the crew to descend to a lower level, get oxygen masks on, manually adjust the landing altitude (trying to recall off the top of my head) and land at the nearest airport. If it is the outer panel, I think you could continue to your destination.