Jerry Coyne has asked his readers not just to wish Dawkins a happy birthday, but to explain how he has influenced them. I think that's a very good idea.
One of my most vivid memories from my first year as an undergraduate at UC Irvine involves sitting for hours on a bench, in a part of the student center that no longer exists, absolutely riveted by replicators and hawks and doves and evolutionary stable strategies. I was, of course, reading The Selfish Gene. Dawkins taught me that evolutionary biology was orders of magnitude richer and more fascinating than I had ever before imagined—and I was a biology major at the time! His penetrating analyses and his gift for language helped urge me along from a premedical focus to the more natural, theoretical focus that culminated with my switching full-time into philosophy of science. Dawkins was not, by any means, the principal catalyst in this change, but everyone who helped kindle my burning fascination with the deepest questions about the way the world works contributed, and Dawkins definitely ranks among them.
Beyond that, what can one say, but what Dawkins already knows—that he is the most talented communicator of science since Carl Sagan, that his public defense of atheism brings solace to many who were formerly voiceless, that he is truly a great man, far more worthy of attention than the silly celebrities and brutal conquerors upon which everyone bestows their plaudits?
Happy Birthday again, Dr. Dawkins, and many, many more.