Darwin's finches don't ruffle creationist's feathers. They really ought not. What Darwin saw was what creationists call microevolution, adaptation due to genetic drift and natural selection. I've never met a creationist who denies this. I've never even met a creationist that doesn't know that mutation occurs, but they do say that beneficial mutations do not occur enough for macroevolution--what they call the formation of new species.
Modern biology doesn't even make the distinction between macro- and microevolution, which is appropriate, in terms of developing the science. However, this is harmful in talking with creationists, because most of the cases of evolution cited by evolutionists creationists explain in terms of microevolution.
One time, when Matthew and I were young earth creationists, we showed up late at a talk and discussion on evolution on campus. I asked the speaker for an example of evolution; her work is in physical anthropology, so she talked about the proliferation of genes tied to schizophrenia. Never mind that schizophrenia is a bad thing, unlikely to sell evolution; when we asked her if we could tell at one point the genes for schizophrenia were not there, and then they were, we got an answer about how the genes spread, when we were really interested in knowing if new genetic information had been introduced. This just made us more convinced creationists, because she didn't even realize what we disagreed with her on. Ironically enough, Matthew and I saw this as a cop-out fueled by cognitive dissonance. Then, we talked until late at night about complementarianism, but that's another story.
A recent article in New Scientist helps refute creationism by addressing its actual claim: that helpful mutations do not occur enough to form new species. The article talks not about a whole new species, but it does talk about how, in the lab, a biologist, Richard Lenski observed a beneficial new gene occur by mutation in e. coli, where it hadn't been before--macroevolution. The ones after a certain point had a mutation that allowed them to metabolize citrate, while the ones before this point didn't have that gene. Bob Holmes, the author of the article, did an amazing job of explaining exactly what happened, so that the creationist dismissal of macroevolution does not hold.