I haven't read all of the replies, but up until recently, we had 2 cats (1 went to live with grandma b/c I am very allergic and she won't stay out of my bedroom!) The cat that we gave to my mother-in-law was VERY interested in watching our chickens and did look like she was in hunting mode. However, I don't think she'd be dumb enough to actually try to take a full-grown chicken down. In fact, anytime they started wandering too close to her, she'd run away looking pretty terrified. Our other cat, however, is a big time hunter and pretty fearless. She's attacked our neighborhood possum and feral cats many times. She completely ignores the chickens, but brings us plenty of mice, voles, chipmunks, etc, as gifts. I do have a couple chicks in a homemade brooder right now, and I have it covered in aluminum foil to deter her. I have no reason to think she'd ever attack a full-grown chicken, but tasty little chicks, perfect size for a snack, are another story.
(Yes, cats hate aluminum foil! My midwife taught me the aluminum foil trick when our cats wouldn't stay out of the baby's things when we were preparing for the birth of my 4th daughter. A sheet of aluminum foil in the bassinet, on the couch, whatever, will keep the kitties off! No idea why they hate it, but they seem disgusted by how it feels on their paws. lol)