I have no problem w/ flip flops, as long as they're not really raggedy. A Birkenstock Medina is the ideal chic flip flop, but those Old Navy ones are pushing it. Havainas are the most common and are acceptable.
But flip flops when the weather is below 75 degrees is insane, in my book. As much as I would love to wear my flip flops in the 50 degree weather it doesn't make sense...From a frozen-foot point of view.