Yes, it is in the Bible. it is not the mark of Satan, persay, but of the 'beast' or the antichrist. It is in Revelations, and it says that in the end times the antichrist will forbid anyone to buy or sell unless they have the 'mark of the beast' or 666 on their forehead.
Now, alot of people today dont believe this to be some kind of tattoo, but more of a tracking device, such as a microchip or PIN that keeps track of what people are doing, buying, or selling, and what.
And no, I am not scared of the number. (Not that I would go out and get it tattooed in my skull.....) But I am, however, never getting a government mandated number or liscence or microchip, that would track where I go and what I do or buy. There are some people that won't get SS numbers for this reason, but they dont track your activity or buying, etc, with a SS number.
National Animal Indentification System, anyone????
Sorry for the long post, I hope this answers anyone's questions a little bit!