Accepting Evidence, Not Faith, as the Arbiter of Truth
The most fundamental and important concession which creationists have made to reality and to science is to accept evidence rather than faith as the final arbiter of truth. Creationism is a religious ideology which is founded on faith, but you'll consistently see creationists trying to make arguments from evidence — that evolution doesn't have supporting evidence, for example, or that the evidence supports creationism.
Only rarely will you find a creationist saying something like "my position is based on faith and I'll hold it regardless of the evidence." Instead, they are far more likely to be seen saying something like "there is far more evidence supporting creationism then evolution; indeed, evolution requires more faith than creationism."
This concession is important because it takes the decision between evolution and creationism out of the hands of theology and places it into hands which creationists cannot directly control or manipulate. Evidence is the domain of science, which means accepting evidence as the final arbiter of truth is the same as accepting science as the final word on the matter.
Accepting that Micro-Evolution Occurs
The most interesting concession to reality and science has to be the acceptance that micro-evolution occurs. It would be impossible to deny the reality of micro-evolution forever because the reality of it is obvious even to the most uneducated eye, but the acceptance of micro-evolution is devastating to the creationist case in ways that they are loathe to admit.
No matter how much they try to deny it, the distinction between micro-evolution and macro-evolution is not a matter of fundamental essence or nature. Instead, both are ultimately product of the same natural processes. Thus the concession that micro-evolution occurs is the concession that the natural processes behind macro-evolution are real. This, in turn, makes it impossible to reasonably and honestly deny macro-evolution. The acceptance of micro-evolution is thus the acceptance of everything creationists want to oppose.
Old Earth Creationism
The oldest concession made by creationists to reality and science is Old Earth Creationism. Young Earth Creationists believe that the earth is only a few thousand years old and are thus in complete denial of most fields of science. Old Earth Creationists concede that a literal reading of the Bible is not an accurate guide to geology and physics.
Conceding this, though, makes creationism more difficult to defend. First, it allows for a long enough time for evolution to occur. Second, it concedes the reliability of several scientific fields which provide multiple lines of evidence that prove evolution occurs. Young Earth Creationism may be a more extreme form of denial of reality, but it tends to be more internally consistent and better able to deny evolution.
Limiting “Design” to just Complexity
The most self-contradictory concession to reality and science has to be the driving premise behind Intelligent Design: that "complexity" is evidence of design. I get the impression that creationists and religious apologists are very impressed with this argument
, but it's appeal is only superficial at best. Once you think about it a little more, serious flaws are revealed.A key problem with this argument is that creationist position is supposed to be that everything was designed — not just "complex" life, but also simple rocks. If complexity is such great proof of design, then the implication must be that non-complex things are not designed and this contradicts the primary creationist position.
On the other hand, if even rocks are complex enough to be designed, then the argument that life is "too complex" to not be designed is undermined. Either way, creationists contradict themselves. They should have stuck to arguing that everything shows equal signs of having been created instead of trying to limit their arguments to just complexity.
