Apple's APIs pretty much make this impossible... They don't want to allow random code to run periodically in the background for fear that some misbehaving app will drain the battery and the user won't be able to figure out which one it is. In addition, this method would require firing up the GPS chip in full-power mode to obtain a position fix. Not as power-intensive as running the chip continuously, but it does use more juice than alternate methods.

chrisch: you may want to consider using the new "geofencing" APIs on CLLocationManager. Specifically: startMonitoringForRegion:desiredAccuracy: or startMonitoringForRegion:. The idea would be to setup a geofence around the user's current location with a radius of, say, 5 miles. When the user goes outside that radius your app delegate gets a notification (via locationManager:didExitRegion:) and you could then record their new position and setup a new geofence around their present position.

Using this technique would allow you to "drop pins" at regular intervals along their route. You could make the size of the geofence configurable: on a local ride you might want pins every mile or two while on a cross-country trip you could get by with 10- or 20-mile intervals.