@atariksi - Why do people say "holy cow" is that in relation to the sacredness of the Cow in the Hindu faith? I.E. the mother bearing milk aka spiritual sustenance?
There's two reasons I see for cow being holy-- one is that its products (incense, milk, curd, etc.) are used for holy (religious) purposes. And two (related to reincarnation), the animal is in the higher species bound to being born as a human having exhausted/atoned for the sins through the various species. So a pure animal in goodness.
I guess they use it as an expression of surprise that a cow is holy or even it's stool is holy given all other stools are smelly and useless. Hey, it even makes sense to say "Holy smokes!" given the incense gives off smoke and used for holy purposes.