A Better Geolocation Prompt

Derek Featherstone makes a good point: The current implementation of the geolocation prompt on most browsers does not give enough information. Wouldn't it be nice if we could provide more information to the user as to why we need this information from them? I would love to see something in JavaScript like this:

if (navigator.geolocation) {
    var geolocationApproved = 
      navigator.geolocation.prompt("This information will be used to provide you with directions to xyz.");
}

would produce something like this: