"Evolution is just a theory. It can't be proven" (I realize Dak addressed this point above, but it can't be clarified enough, sadly.)
1. Its both a fact and a theory. Populations of organisms change over time. This is a fact. Evolutionary theory is a cohesive attempt to explain how this fact happens.
2. A theory is of course, not simply a guess. Theories are essentially works in progress arrived at through rigorous testing.
3. Since theories are works in progress, they are indeed not proven. They must remain so to make room for new discoveries.
"There is no way life could have evolved as it is through pure chance."
Chance does indeed play a big role in evolution, but natural selection, it's most important mechanism, is not random.
"Evolution violates the second law of thermodynamics."
The second law of thermodynamics states that entropy (disorder) cannot decrease in a closed system. Since an organism is not a closed system (closed systems are merely an ideal) as energy from the sun comes and goes, evolution does not violate this law.
"Creationism should be taught in public schools as an alternative to evolution."
1. Creationism is essentially an aspect of Christian religious scripture. Thus, teaching it in school is in violation of the American constitution.
2. Science is not a democratic process. There are no "sides" of a story with scientific theories. The one that best fits the evidence wins, thats it.
"Ockham's Razor states that simpler theories are more valid over complex ones. Evolution is complicated; creationism is simple."
This is an oversimplification of Ockham's razor. It actually states that the theory with the least amount of terms is valid between two or more competing theories that make the same predictions. It is essentially a way to remove redundant terms in an explanation.
"Evolution is ruthless and leads to immorality."
1. Nature is indeed ruthless. Thats just the way it is. Welcome to the real world.
2. Scientists don't pass any moral judgement on the phenomena they describe. If it leads to immorality, it doesn't mean that evolution isn't a valid theory.
"Debate continues about evolution. Doesn't this mean it's not a good theory?"
Scientists don't debate the fact that evolution occurs. All debate centers around the finer aspects of the theory. Dissent and debate among the scientific community is healthy and leads to the sharing of new ideas. This is a good thing.
"There are problems with evolution, so the whole thing is wrong and creationism wins."
1. No one disputes the fact that there are things we don't yet know. However, if science simply admitted defeat every time a mystery was encountered, there would simply be no such thing as science.
2. Theories are not airtight. Thats why they are called "theories". Theories are works in progress.
3. Anything we don't yet know is not proof-positive of divine intervention.
4. If evolutionary theory was falsified tomorrow, creationism wouldn't be viable automatically. There may be more than 1 other option.
"You are being closed-minded for not considering creationism."
Being closed-minded about theories with no evidence is a healthy scientific mindset. Are modern evolutionists also close-minded for not accepting Lamarckism?
"So and so, PhD is/was a creationist. If he/she could believe it, it must be true."
Appeals to authority don't prove anything. Opinions an individual may hold can be wrong. Aside from this, 99% of scientists accept evolution, so one can just as easily (and more effectively) argue for evolution with this method.
"You know the Piltdown man was a hoax."
Yes and it was proven to be just that 50 years ago. It is no longer considered anything but an embarassing moment in bioanthropology textbooks. I might add it was also revealed by other scientists, which is a good example of how science works. Scientists are human; they make mistakes, get excited and leap to conclusions just like anyone else. But they will usually admit they are wrong and everyone moves on.