I should have stressed that being USB-integrated was definitely a "nice to have." I need a range of at least 150 feet, but preferably somewhere around 500 feet; that's why I originally ruled out RFID, but if it's possible now to get a good reading/signal from that distance then I'd certainly look that direction.
The receiver will be positioned inside a building while the mobile device will be outside the building. For instance, say I needed to detect when a certain known vehicle approached the building; that's basically the situation I'm looking for.
[edit] As long as the reader is strong enough, the mobile device does not NEED to transmit anything; I just assumed that because of the distance requirement the communication would need to go both ways.