The altitude information would be returned if it's available. The location API that powers this uses several different methods to determine a users location; GPS, WIFI triangulation, IP address. Only the GPS method would be able to provide the users true altitude. Wifi triangulation could provide an approximate one but is generally not returned due to error.
If you want to get the elevation of the ground for where the user is then this could be retrieved from the Bing Maps Elevation service: http://msdn.microsoft.com/en-us/library/jj158959.aspx