They do point out to a relevant history that in 2003 a similar kind of 'last day of the Earth' rumor went afloat but nothing happened and so the date was further moved to Dec 2012. This date is being taken much more seriously by a majority number of people and off late a number of stories regarding spiritual gurus, who are preparing for the D Day, is circulating. Stories for people preparing for the last day is also making the grapevine.
The NASA scientist specifically talked about the Mayan calender. Many people believe that the calendar doesn't have any more dates after December 2012, hence the spook factor but according to an article in ndtv.com, NASA said, “Just as the calendar you have on your kitchen wall does not cease to exist after December 31, the Mayan calendar does not cease to exist on December 21, 2012. This date is the end of the Mayan long-count period but then - just as your calendar begins again on January 1 - another long-count period begins for the Mayan calendar.”
As far as the Niribu planet is concerned, it was allegedly an Internet hoax. They say that if such a planet had an existence, as it is claimed, then the planet in question would have been traced by astronomers for years and it would also be seen by the naked eye by now. The astronomers of NASA are also carrying out a Spaceguard Survey to find any large asteroids near Earth long before they actually hit. When the dinosaurs went extinct, there was a huge collision and it happened 65 million years ago. This time no such asteroids are making a headway towards Earth- that they know of till now.