I have never heard the theory that the moon will one day actually crash into the earth. In fact the moon is moving away from the earth at about an inch a year. Over a lifespan this is not too noticeable, but over a course of a million years things like tides will be weaker. Check out information at Astronomy magazine's web site:
http://www.astronomy.com/asy/default.aspx?c=a&id=2195