How does this impact the theory of evolution?
Beyond that, your objection basically boils down to a statement that it's impossible to tell the difference between designed and non-designed biological phenomena. Of course, this cuts both ways: if you can't tell the difference between design and non-design,
You cannot tell me if the world was created last tuesday either. However, that does not fall within the realm of science. Science makes predictions from evidence and observation. And the evidence is that the universe is billions of years old and evolution took place here on Earth. Anything else falls within the realm of faith or a belief system.
then evolution isn't science, either -- how could it be, if the fundamental basis of the theory is, by your own assertion, untestable and unprovable?
Nope. There are predictions that can be tested for evolution. This puts it squarely within the realm of science. ID cannot make such a claim.
Wrong. The validity of ID as an explanation for biological phenomena is easily testable, and I'll give you proof: you can buy stock today in a biotech firm of your choice (and there are many, many choices available). From this, we must conclude that ID is an experimentally proven concept. Moreover, you'd be hard-pressed to claim that that particular form of intelligent design was not science.
The bottom line is that, regardless of whether or not ID is the actual explanation for phenomenon X, it must be acknowledged as at least a viable explanation. We can say exactly the same thing about the theory of evolution. For example, evolution may be a viable explanation for, say, certain strains of corn, even though the real explanation is something else (i.e., human intervention).
Your basic position boils down to what I indicated previously: that there's no way to tell the difference between designed and non-designed. It's beyond strange that you seem to be saying that this is not a valid field of scientific inquiry.