Cyclocross bikes would also be a good choice for variable commute conditions. I like by Gary Fisher/Trek Lane a lot. I went looking recently for a steel bike, and am very pleased with my Lane. It's a very comfortable bike to ride. Great geometry, smooth ride, and rated for mild off road riding for drops of up to 12 inches. And the drop bars are great for different hand positions.
Not a serious MTB by any means but easily handles the potholes and cobblestones of DC and old town Alexandria.
