This is a must read blog post by Jeff Jonas –
Mobile devices in America are generating something like 600 billion geo-spatially tagged transactions per day. Every call, text message, email and data transfer handled by your mobile device creates a transaction with your space-time coordinate (to roughly 60 meters accuracy if there are three cell towers in range), whether you have GPS or not. Got a Blackberry? Every few minutes, it sends a heartbeat, creating a transaction whether you are using the phone or not. If the device is GPS-enabled and you’re using a location-based service your location is accurate to somewhere between 10 and 30 meters. Using Wi-Fi? It is accurate below10 meters. (Jeff Jonas)