There is an Item type Location and it has a distanceFrom method. So if you have your home location set in a Location Item and the phone location in a Location Item its as simple as:
if(Phone.distanceFrom(Home) as Number > 100)
I do not know whether the location built into OH is available to be used to populate an Item, but you can use Design Pattern: Encoding and Accessing Values in Rules to create and initialize one.