Obviously, if Natalee has had disappearance issues in the past, her family would likely keep them quiet since it would surely impede the investigation -- just as the Wilbanks family probably figured when they kept quiet about their daughter's past escapades.
Oddly, the longer the goes on, the more I'm inclined to think the outcome won't be as sensational as we might have thought in the beginning.
By now, if this girl were a runaway someone would have said something. She can't have acted totally alone and be hiding out without food, money, clothes, etc. all by herself somewhere. (Wilbanks was in her own country, took some money, knew where she was going, etc.) I doubt Natalee would be able to be resourceful enough in a strange place. Also, if someone had helped her get away, they would have talked by now, because the pressure is just too great. No one would cover up for a girl who had only been on the island such a short time and was unknown to anyone except a few fellows. No one would put themselves in that kind of jeopardy.
Sorry, IMHO the Runaway theory just does not "compute," but there are so many others FReepers have come up with that are possibilities. Who knows?