Over at El Goonish Shive Dan just posted "I've heard many weird, crazy and strange theories regarding evolutionary reasons why animals do things, but one of the coolest reasons theorized for why cats hiss is that they're trying to convince you that they're like a venomous snake." Now, this sounded odd to me, but I figured it was just some of that good old fashioned urban-legend/popular rumor that happens. Except at least one person with a PhD in Zoology seems to buy it as well. Of course, it's important to note that A) Desmond Morris seems to have a pretty wide ranging portfolio, having made his mark originally with The Naked Ape and as a TV host in Britain, and B) he couches the explanation as "It seems likely... It has been claimed... if... and this is probably the true explanation of the way in which the feline hiss has evolved."
STILL, THOUGH! I call shenanigans! Because I've got a cockatiel and he bloody well hisses, too. It's tiny and cute and doesn't really sound right because his mouth is built all wrong for it, but it's still a hiss. And so do many other birds. And dogs growl, which is still a noise made by expelling air when angry, it's just done with the mouth closed rather than open. And, for that matter, *SNAKES* hiss! Who are they imitating? Also, cats also arch their back while hissing to make themselves look bigger. This looks NOTHING like a snake.
I'm not saying that I have any better answer except for the more general one: "Most animals, when threatened, make some noise as an almost universal signal for 'You might kill and eat me, but I will NOT go down without a fight. So BACK OFF!" But personally? I think the resemblance of that noise between cats and snakes is more likely coincidence.