"Evolution theory has not been proven true, despite more than 100 years of trying. That says it all, folks."

Short answer: none of science has been proven true. It has only been proven highly accurate: a seemingly small but philosophically important distinction which creationists take advantage of.

You see, there is no such thing as a scientific proof of truth. As the philosopher David Hume pointed out, we cannot even prove that the Sun will rise in the East tomorrow! Yet we are still highly confident that we can turn to the East and see the dawn tomorrow, are we not?

If you are interested in a philosophical rebuttal to this argument, look up the writings of "Karl Popper". But if you are interested in a more straightfoward explanation, let's just say that it's far easier to disprove an idea than it is to prove it.

Think of the truth as pi. Can we ever know whether any given number is exactly pi? Probably not. No matter how accurate it is, there will always be some error. But does that mean we can't look at the number 2.1 and say "that's way off the mark"? You can never know for sure that any given scientific theory is perfectly true, but you can test to see if it is false, or so far off the mark that it should be stricken from consideration.

Has evolution theory been "proven true"? No, but it has been proven highly accurate, and like it or not, that is the most that can ever be expected of any scientific theory. To seize upon this fact as a weakness of evolution theory is to betray either willful or negligent ignorance of the way science (or for that matter, the pursuit of knowledge in general) works.

Fallacy watch:

• "Appeal to Ignorance" fallacy (since evolution theory has not been "proven true", they assume that it must therefore be false).
  
• Strawman distortion (they pretend that scientists have been trying and failing to prove evolution theory true).