The American Heritage Dictionary of the English Language, Fourth Edition states that although the word "kudos" looks like plural, it's really not. Since kudos has been used as a plural so often in common usage over the last few years casually and in the press, a new singular usage has been accepted as proper. The singular form still isn't very often used as a descriptor, with most people preferring the original "plural." In fact, based on similar word derivations, the pronounciation should leave the last 's' sound unspoken anyway, as in "pathos."
Other examples of words that have suffered this fate include congeries, pea and cherry. The last two, interesting enough, were shortened from the old English "pease" and the French "cerise."