I am new to Indigo (new as in still on the trial license new ) but I just submitted my first plugin that integrates with Google Latitude, creating variables representing the distance (in kms), latitude and longitude of each user (you can define as many users as you want). You can find it on http://www.perceptiveautomation.com/filelib/index.php filed under Application Integration.
It requires registering and creating a unique URL on https://latitudebridge.appspot.com which allows you to have a single curl-able URL to fetch the current best location of a user (the app supports iPhone, Blackberry and Android at least, see http://www.google.com/mobile/latitude/).
Now, I can finally have it send me an email when the garage door is open while I am away. It also turns off unnecessary lights when I am away for an extended period of time. Technically, you can activate your vacation schedule when you are gone too.
P.S. The script has been updated to add some try .. end try blocks to prevent connectivity issues from killing the repeat loop. That should be up in a bit (pending the admins to update the page).