I make my 6 mile commute in regular work clothes. Memphis doesn't get super cold. 31-degrees has been my coldest start so far. I wore a moisture wicking base layer under a merino wool cardigan, regular chinos and work shoes, a wind breaker and knit gloves. It worked out fine, except it was a pretty breezy day and my ears were really cold. I need to get one of those insulated caps with ears that goes inside the helmet.
I'd like to start commuting in dresses and skirts (right now I change at the office if I want to wear a dress/skirt that day). I learned a neat trick from a video someone posted of Dottie over at the Let's Go Ride a Bike blog. On extra cold days (she's in Chicago) when she's in a dress, she pulls a pair of wool leggings over her regular tights and then takes them off when she gets to her destination. I'll have to try that next time.