I heard a theory once, that the domestic cat's "miaow" developed simply as a form of communication with humans. They've certainly learned our rules of intonation; listen carefully when a cat REALLY wants some attention - it can sound quite pitifully pleading!
I also heard that cats do not meow at other cats, but only at humans. They do seem to use different meows for different things, as my cats will often meow in a happy, upbeat tone when I say hello to them, but if they are hungry and I ask them in a 'sympathetic' tone what they want, they meow at me in a slow, slightly croaky, whining tone!