Logan Gage at evolutionnews is wondering "What Exactly Does Genetic Similarity Demonstrate?". Given the 98.8% DNA sequence similarity between chimps and humans, Gage writes:
Some design proponents think the evidence for common ancestry is good (e.g., Michael Behe), while others—citing the fossil record, especially The Cambrian Explosion—do not. But neither group thinks that sequence similarity alone proves either common ancestry or the Darwinian mechanism, as so many science writers of our day seem eager to assume.
That last sentence is nothing but a straw-man. The strength of the evidence regarding evolution does not rely on any one particular field of study or observation but it is the combination of evidence, often from hugely disparate fields, that supports the notion that all extant life-forms share common ancestry. What Gage seems to be forgetting is that the idea of common ancestry didn't start after DNA sequencing became common practice (heck, he even mentions fossils in the previous sentence), but actually predates it by at least 100 years - and Darwin wasn't even the first person to propose it. Although not making any suggestions of common ancestry, Carl von Linné set the stage by classifying living organisms based on purely phenotypic criteria. It wasn't a huge leap of the imagination (by today's standards, anyway) to suggest that the similarities found between different organisms was due to common ancestry. The sequencing of genetic material - including entire genomes - has confirmed what we already suspected - common ancestry. Using evolutionary theories, we would have expected the DNA of phenotypically similar organisms to be similar. The case for common ancestry was, therefore, strengthened. Using ID theory, we would have expected... well, we wouldn't expect anything in particular. Even if all organisms share common ancestry, there is no reason, under an ID theory sceneario, to predict that phenotypically similar organisms should also be genotypically similar. After all, "the Designer" might have changed the DNA to whatever he/she/it wanted. And no, this suggestion does not go against ID and there is NO reason for ID supporters to think it preposterous. Remember, ID theory says NOTHING about the designer, so it is quite a plausible ID scenario if we speculate - as we can - about the "mysterious ways" of the designer. No wonder the ID folks can't agree on whether they believe that all extant life forms share common ancestry.
Summa Summarum: No scientifically minded person claims that DNA sequences alone "prove" common ancestry - although it certainly strengthens the case.