Carlton Reid pointed this out for us: Mackinac Island in Michigan has no motorized traffic (except for emergency and service vehicles), and has the only State Highway in the United States with no motorized traffic.  Check it out on Wikipedia.com or this little website.
They allow walking, bicycling, rollerblading and skating, but there are also horse drawn carriages, so watch for land mines.
Also via Reid’s knack for finding bicycle related goodness is this list of car-free places, also via Wikipedia.
