From the experts:
Varying the spelling of an established name or word to substitute a k for a c has become more and more kommon, oops, common. Some parents feel that using a different spelling makes the name more of a name and less of a place or a word. Kairo may seem to lend itself particularly well to this practice given that the K spelling turns the first syllable into the stylish name Kai. But we prefer the more authentic Kai or Cairo.