Based on demo here: http://www.phonegapessentials.com/content/chapter-19-geolocation-api.html
More up-to-date info on geolocation here: http://docs.phonegap.com/en/2.4.0/cordova_geolocation_geolocation.md.html
###IMPORTANT !### To make geoLocation work in PhoneGap you MUST add the following line to www/config.xml:
<plugin name="Geolocation" value="org.apache.cordova.GeoBroker" />