This lightly insulated Trekkin skirt from Mountain Hardwear just might become the go-to cycling item of your winter cycling wardrobe, as it works well over tights, and has enough room to fit on top of or underneath rain pants.
The skirt is 16.5 inches in length, so will fall at mid-thigh on most people, making it just about perfect for covering your tush but not too unwieldy even for diamond-frame bicycles.
It's got that bike-to-boardroom feeling, and is waterproof. Side zipper.
Available at the Terry Bicycles web site along with a nice selection of fashion-conscious commuter cycling wear. ($100)