No, Armageddon is not starting today and the rapture is not going to take place.
The sad thing about this is that the Bible is actually full of surprisingly accurate prophecies that have been confirmed historically. Unfortunately people are following their own unbiblical methods of Bible interpretation, resulting in such queer eschatology as that of Mr Harold Camping, who predicts the start of the end of the world will begin today.
Also, the idea of a rapture apart from the Second Coming is foreign to the Bible; especially the idea that the "church" will be spirited away and spared from the Great Tribulation. No, the Bible, if actually read properly, does not suggest that Christians will be taken out of the world when the Great Tribulation occurs; just like the Israelites were not spirited away before the plagues hit ancient Egypt. The Israelites only left Egyptian captivity after the plagues and tribulations. Christians sealed by God are promised protection from the plagues, but no mention is made of some kind of rapture that takes them out of the time of tribulation. Such theories are based on the idea of dispensations which is another concept foreign to the Bible, but believed by many evangelicals. God is ". . . the same yesterday, and to day, and for ever" (Hebrews 13:8), there are no dispensations in which God relates to us differently. The covenants have always been based on the same principle of God's grace.
I fear the followers of Mr Camping will experience a "great disappointment", which will unfortunately give his (atheist) opponents fodder for mockery, not only of Mr Camping's and his followers, but of all Christians.
If only they had read Matthew 24:36:
"But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only."
A more unfortunate effect is that these false prophets are crying wolf, so that when the true warnings go out, the hearers will not take head. My heart goes out to these people that will be sorely disappointed today and the mockery they will have to endure in the weeks and months (and years) to come. Some of them will give up their faith completely; hopefully others will not give up on their search for Truth.