One of the favourite claims of the ID/creationist crowd is that evolution and the big bang are "just theories" and that since theories have historically been replaced many times, there's no reaons to give any special credence to these inconsequential bits of short lived belief.
Fundamentally, this shows an enormous understanding of what constitutes a scientific theory. One of the most frequent ways that I respond when debating these pseudo-scientists is to remind them that "gravity is just a theory too."
Inevatibly, I get one of two responses. Either they deny that gravity is a theory, or claim that the evidence for it is overwhelming (which ironically, it is for evolution and the big bang as well) and thus, it's the best theory ever and some how not comparable to the other two.
But how sure are we about gravity?
The first major understanding of Gravity came in the Philosophiae Naturalis Principia Mathematica, written in 1687 by Isaac Newton. While people before then were assuredly familiary with gravity, Newtons workings were the first mathematical and scientific treatment of the topic.
Newton's work was a huge success in that it successfully explained a huge number of occurances. But perhaps most excitingly (at least to me) was it's ability to predict things that were far beyond the everyday understanding.
In the 1840's astronomers had been conducting careful observations of Uranus' orbit and noticed that it wasn't quite right. This meant that either Newton was wrong, or there was something strange going on. After careful analysis, two scientists, John Adams and Urbain Leverrier, independantly realized that the oddities could be explained by another, yet undiscovered planet. Additionally, they were able to predict where this planet should lie. Sure enough, Neptune was discovered!
(This is good science folks: Predictions made and verified!)
However, by the early 1900's, it was clear that Newton's theory had some problems. One of the main reasons was that the orbit of Mercury didn't "wobble" like it should.
Enter Einstein! In his most famous work, Einstein completely overhauled gravitational theory and fixed all the problems of the time by postulating that it was a curvature of space time. Several experiments later, Einstein was, like Newton, was vindicated, and the theory accepted.
However, 100 years later, it seems there may yet be room for revision!
Another favourite tactic of ID supporters is the "teach the controversy" mantra. However, in the scientific field, there is no statistically significant debate. But the same cannot be said for gravity. Currently there are several models working to append or supplant Einstein's work.
One of the major realizations is that, under extremely high energy densities (like what occured right after the big bang and today in particle accelerators), the four fundamnetal forces known (the strong and weak nuclear force, electromagnetism, and gravity) should all become a single unified force. However, while theories have been developed that relate the first three, gravity has been the odd man out and hasn't quite fit in.
In fact, while the other three forces are all similar in strength, gravity seems to be strangely "weak". Why gravity isn't as strong as the others is currently one of the unknowns. Thus, the newest theories working on fitting gravity into the bigger picture have suggested things like other dimentions which we cannot percieve that gravity can "leak" into.
So there's one major addtion to gravitational theory that's being reviewed.
But is it the only one? Not at all! Perhaps you've heard of something called Dark Matter? If not, it's a theory that states that ~3/4 of all matter in the universe is invisible and not interact with light for some reason. That means we can't see it giving off light, or absorbing light. So how do we know it's there?
That's where it relates to gravity. The first realization came by studying the orbit of stars around spiral galaxies. Under all current gravitational theories, it should work pretty much like the solar system: Stars close in orbit quickly whereas stars further out move slowly.
But observations say otherwise. Strangely enough, stars further out move almost as quickly as those further in. This has generally been interpreted to mean that there must be some more mass hidden somewhere.
However, that assumes that the gravitational models are correct. It's also possible that gravity may behave differently on such a scale. Thus, a modification to gravity has been proposed known as Modified Newtonian Dynamics (MOND). This hypothesis adds an extra term to Newton's equation for gravity that takes scale into account. Depending on how it's done, it can completely eliminate the need for dark matter, and possibly the even more enigmatic dark energy.
So as you can see, even the most universally accepted theory isn't immune to scrutiny when presented with evidence that runs contrary to the model. Meanwhile, evolutionary theory has yet to have a serious scientific challange levied agaisnt it.
Various unified models of gravity (espcially string theory) and MOND have a fair share of support in the scientific community. Surely far more than Intelligent Design, yet we don't ever see the ID crowd clamoring for an actual scientific controversy to be taught, only their own pet hypothesis. So much for intellectual honesty on their part.