This Applescript checks to see if you're away from home and sets OS X to require a password if you're out. If you're at home, no password is required. It requires Location Helper to work (free on the app store). I also recommend Lingon to make it easy to setup the repeating task in launchd. Just set your home address and run this periodically.

away_from_home.applescript

tell application "Location Helper"
  set home_address to geocode address "123 Main St., Anytown, US"
	
	set location to location of geometry of item 1 of results of home_address
	
	set home_coordinates to {lat of location, lng of location}
	
	set distance to get distance from coordinates home_coordinates
	
	if distance > 500 then
		set away_from_home to true
	else
		set away_from_home to false
	end if
	
	tell application "System Events"
		set require password to wake of security preferences to away_from_home
	end tell
end tell