I have commuted with old Raleigh singles speeds (coaster brakes) and "single speed" bikes with conventional hand brakes. I love it. One of my 10 speed 26" wheel bikes is permantly stuck in 2nd gear right now so it's kinda like a "single speed"...

As soon as our local "Crappy Tire" store (my fellow Canucks know what I mean) has their old school coaster brake bike on sale for $99 I am getting one for winter commuting. There aren't a lot of hills where I ride.