Obama's DNA could be compared to Malcolm X's living children, but we know that's never going to happen.
As far as the theory that MX could be Obama's father, I think it holds as much water as any other theory. Per what I've read of Stanley Ann Dunham's history, she was in the same vicinity as Malcolm X for a brief period, at least nine months prior to Obama's birth. That possible connection has been explored here many times.
I take your point that totally unrelated people can strongly resemble each other, but that's really a rare occurrence, and shouldn't be used to try to prove that Obama isn't related to MX. Factually, most people bear a stronger resemblance to their parents than most others in the general population.
No, resemblance between two people isn't proof positive that they're related, but it can definitely weigh as circumstantial evidence that they are, in cases such as this. This is why you're going to continue to hear people bring up MX as a likely candidate for Obama's real father.
Malcolm X was in Seattle briefly at the same time as was Stanley Ann Dunham. That she would run to him to impregnate her while he happened to be in town strikes me as very unlikely, though groupies do exist. Even so, the numbers don't work (if the official birth date is true) because she became pregnant in November-December of 1960, long after they had left Seattle.
I take your point that totally unrelated people can strongly resemble each other, but that's really a rare occurrence, and shouldn't be used to try to prove that Obama isn't related to MX. Factually, most people bear a stronger resemblance to their parents than most others in the general population.
Everything about Obama is a "rare" occurrence. What's one more amazing coincidence? :)