I googled the same thing yesterday (somewhat bored at work) and there isn't a definitive answer as to why 666 is the number of the beast. There are two major schools of thought, one of which is that it's code for Emperor Nero, who I guess was unpopular at the time of Revelations being written, and this is the theory supported by the Catholic Church apparently. I can't remember the other one, but there's all sorts of stuff about it on wikipedia.
Today's Independent leaps enthusiastically on to the 666 bandwagon, and the article includes this background: You find the origins of 666's bad rep in the final book of the New Testament, the book of Revelation - in its visions of the destruction of the world. Satan is defeated, the last trumpet sounds and Christ triumphs.
The literal interpretation of Chapter 13 is that the presence of the Antichrist, a false prophet sent by Satan, marks the end of time - that there will be a period of "trials and tribulations" before the expected return of Jesus.
The Antichrist will attempt to win supporters through miracles and will kill those who refuse allegiance. The Antichrist's rule will be totalitarian: the mark of the beast is imposed on everyone, and "no man might buy or sell, save that he had the mark".
The key passage reads: "Here is wisdom. Let him that hath understanding count the number of the beast: For it is the number of a man; and his number is six hundred threescore and six." So begins the hunt for the Antichrist, in order that he may be known and defeated. Hence the superstition about 666 entered Christian folkore.
However, this is, liberal theologians point out, a fundamental interpretation of the Book of Revelation.
As the Anglican priest and author Revd Lionel Fanthorpe puts it: "It is important to place the Book in historical context. If you or I were Christians in the first century, when Nero was persecuting Christians, then we would have been rather foolish to carry a book attacking the emperor by name.
"So they used code: the mark of the beast, 666, identified Nero, who they believed would be judged and destroyed by God. This was written to comfort the persecuted Christians that God would save them. Other readings are a misunderstanding."
It goes on to say it's been translated wrong, and the number is actually 616, which rather ruins the whole thing...
Also, today's date is of course made up way after the thing was written, so as predictions go, should it be the Gregorian 6/6/6 or the Julian calendar's 6/6/6 or the Jewish calendar [drones on for an hour...]. All twaddle. But fun to discuss, obviously!