Armageddon/WMG

This movie makes perfect scientific sense.
...at least as far as the asteroid is concerned.

Listen close. The "anomaly", aka the asteroid in the film, is a large fragment of what was once a Pluto-sized dwarf planet. A few decades before the movie, two dwarf planets collided and shattered into pieces, with the anomaly being one of the largest chunks. This happened sometime before 1970; when it happened, either we didn't have the technology to see it (no space telescopes, etc.), or (more likely) we didn't realize what it was and why it would eventually be important.
 * "And beg'n your pardon sir, but it's a big-ass sky." It really is a big-ass sky. For fun, let's assume that the asteroid hits the Earth today (early-mid 2010ish) and that we detected it when Armageddon came out (1998). That would mean that it's 7.05 × 10^13 miles away. Now Texas is 773 miles wide. And object the width of Texas, at that distance, would be .0000000000000001322% of the sky (the area of a 773 mile wide object on a 12-light-year sphere). Scaling math is left as an exercise to the reader (Surface area of a sphere is 4*PI*r^2, and (assuming a spherical object (i.e. asteroid)) the portion of that sphere it takes up is PI*r^2)

Now you've got a big chunk of molten rock (a collision between dwarf planets would melt both planets) hurtling through the solar system. As the rock slowly cooled and coalesced, friction built up. The rock didn't come entirely back together; it reformed in the shape of two VERY loosely bound chunks, and the rock was riddled with fault lines all over the place (which explains the earthquakes and geysers that the drillers-cum-astronauts had to deal with).

If you're wondering how a simple nuke drilled 800 feet into a Texas-sized rock could shatter it into two equal halves - it was two equal halves to begin with, and friction barely kept the halves together. Hell, it was already coming apart when Bruce Willis's character put the nuke in there. The energy of the nuke was the "straw that broke the camel's back". The force of the explosion was just enough to overcome the friction holding the two rocks together, blasting them just far enough apart to just dodge the Earth on both sides. Gravity will probably bring them back together, but by then the asteroid will be heading on its merry way out the other side of the solar system.
 * It makes a little more sense than that if you listen to the DVD commentaries etc. The asteroid has travelled the galaxy for several million years, which excuses almost all its weird attributes. (Maybe not the grass.) It's full of gas too, and the nuke hits it at a fault line that makes it crack while all the gas go boom. I'm still wondering how they got the halves to fly in the right directions not to hit Earth, though.
 * They can't. The amount of energy needed to push the halves of a rock the size of Texas that far apart when the Earth was already close enough to fill half the asteroid's sky would be more than if a nuke ten miles wide was set off, let alone a burp from a big gas pocket. All you'd get was two slightly separated impact points instead of one. Let's face it, this film just takes place in an Alternate Universe with different laws of physics. Only possible explanation.
 * Yes, they fail physics forever. However, the base idea (splitting an NEO into two halves that both miss the Earth) is viable idea Warning: Do not operate heavy machinery after reading the articles linked But (allowing for Twenty Minutes Into the Future technology to get them there) nothing is overly far fetched outside of visuals. Aside from Bruce Willis somehow being a genetic ancestor of Liv Tyler.

The movie's plot is merely a side-story to the real heroes.
There have been issues raised by people concerning the realism of this movie. A giant Texas-sized boulder would have been spotted decades previously. If one were only 18 days away from impact, it would be visible to the naked eye. Also, one nuke, even planted on "fault line", would never be able to split such a huge asteroid. So the rock was, in fact, warped into the system by super-advanced aliens who wanted to destroy the Earth, or at least the humans on it. But some equally-advanced alien (who likes the Earth) also arrived and was able to split the asteroid using all the powers at his disposal. It just happened to occur right when the human team fired their nuke. As for the visibility factor: Perception Filter. That's right, the movie is a Doctor Who episode, like "Earthshock" but where the asteroid was actually stopped.

The asteroid was, in fact, a Fae lord.
This is why the movie makes no scientific sense - and why the asteroid is so damned malevolent and the gravity is screwy. The asteroid is really a giant Fae lord who wanted to screw with humanity, and when he saw the lengths that they would go to in order to save their planet, he decided to "play along" and split apart anyway because he appreciated how they acted and their brave actions.

This movie and the Transformers trilogy are set in the same continuity.
"Armageddon" is just the name that the media gave to the asteroid events of 1998. When the kid in the first Transformers film says that what's happening is "way cooler than Armageddon" he means that it's way cooler than what happened in 1998. This leads to a corollary: As a related WMG:
 * The asteroid was hurled toward Earth by Unicron. This would explain why it's much bigger than any actual asteroid (Ceres, the biggest asteroid, has a radius of "only" 487.3 km) and why it looked like a chunk of a much bigger body: because it did not come from the asteroid belt! It was actually a chunk of a planet in another solar system, that Unicron destroyed.

The 2013 Teenage Mutant Ninja Turtles movie will be set in the same continuity as well.
Michael Bay said that in his movie, the Turtles will be aliens. I bet that the scenes dealing with their origin will have a Shout Out toward Armageddon, Transformers, or both. And, as an ultimate WMG:

There will be eventually a movie which ties together the plots of all Michael Bay movies.
It will be in 3D and have lots of explosions. There will be asteroids, alien robots, clones, sapient turtles and World War 2 veterans. There will be original elements as well, and it will be mind-blowingly awesome.