My current place scores 17, as contrasted with my apartment in Cambridge that had a comfortable 80. I couldn't remember, but poking around my old DayRunner, I found the address of my apartment I lived in when I moved from Cambridge to Bothell, before I bought this house. That place scores 40.
Wendy suggested that public transportation should figure into the calculations (which I'm sure would be a lot more difficult than what they're currently doing). I'd say not just include public transportation, but take into account the actual path that a pedestrian would walk. Perhaps multiply the distances by 1.5 if it's walking alongside a road with no sidewalks. For my home, it says I'm 4 miles or so from a movie theatre, but that's as the crow flies, and if one were to walk, it'd be probably 8 miles without a sidewalk, and another .5 with, so call it "12.5 normalized walking units".
Seems like beyond about a mile, any distance is practically infinite - people won't walk it at all.