JOEL - We don't hear General Authorities speak about it because the interpretation of that concept has not yet been fully revealed. Many have speculated on what it means and have offered their own interpretations as to what the mark refers to. It could be an obvious physical mark on the foreheads of those who worship the beast(a representation of Satan's power), or it could be just a figurative expression, describing the spiritual condition of such people. Mostly what John was concerned about in his writings(Rev. 13) is the question of worship and to whom men will give their loyalty. Bruce R. McConkie once said about this "mark":

"Reserve judgment on those things for which no interpretation is given. An example of this is the so-called number of the beast(666), which is stated to be the number of a man, which, if it could be identified, would show who was involved in the great deceptions imposed upon mankind. This is an answer that we do not know. The wise course is to avoid being entangled in the specious speculation of an uninspired world." (Bruce R. McConkie, "Understanding the Book of Revelation," Ensign, Sept. 1975, 85)

When it is important for us to know the specifics of this the Lord will reveal it to His prophets.