Is it currently possible to create an app that \"reminds me when I get to work, Siri\" right now ? Or the app needs to be running to apply geofencing ?
Yes, it is possible by using the CLLocationManager's startMonitoringForRegion which will call your App when entered/left a certain "fenced" geo-area.
BUT since the user's location is/will be used, the location icon will appear for as long as this call has yet been canceled (by the matching stopMonitoringForRegion)
(on iOS6 this icon will be replaced with a different location icon for "regional" location instead of regular location use - looks like an outline of the current icon)