Many women I know prefer the more "feminine" look of skirts and dresses even while cycling; my wife is one of them. In the San Francisco Bay Area, I see a handful of women who dress up for work and ride their bikes in their office wear. One in particular I see almost every day dresses fashionably and rides to and from the office on an old white Peugot 10 speed. Even in her straight skirt on a bike with a top tube she manages to ride demurely and confidently.
A common way to stay modest is to wear bike shorts or tights under the skirt. Byron the Bike Hugger posted some links to handy tips for the ladies who might want to ride a bicycle in a dress or skirt. Pauline seeks to excite onlookers with her advice in Happy Woman Magazine in which she advocates stretchy, short skirts. Lisa offers somewhat more practical advice for the more modest cyclist at at the San Francisco Bicycle Coalition. She suggests an elastic band on the thigh to which you can safety pin your skirt to prevent wind-blown exposure. The Guardian notes that long skirts become short skits and short skirts become shorter skirts on a bike. Darcy writes she can ride in anything with no fear.
I'm glad to see more and more people who see cycling as an activity that can be done in any kind of clothing.
Top photo: "Santa Cruz Cyclist" by me.
Bottom photo: "Kirsty" by Hen Waller. See also the excellent Velocouture pool for many more photos of stylishly dressed cyclists.