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TW was correct for the class of tags called passive, and that includes the mobile speedpass readers you can put in your car. Those use an antenna positioned above the gas pump to imterrogate the widget you're supposed to mount in a window. The NY/NJ/PA EZpass tags are active...they have a battery and transmit when asked to...gives them a lot more range. I'm pretty sure the Mobile speedpass car gizmos are passive. I use the keyfob one, and used to have a NYS EZpass. and I design stuff like this for a living.
In theory, you could get an RFID tag to work from many miles (think military radar which identifies targets from a long distance away solely on interpreting echoes, even when the object in question doesn't want to be identified.) The issue is only one of economics: power, range, number crunching, safety.
I have credit cards that are chipped, my company badge is an RFID tag, my cellphone reports GPS position to E911, and a 802.11 LAN can do geolocation if you program it appropriately.
remember that the only time time your cellphone or laptop is "off" is when you remove the battery..all other times they are exploitable by malicious code
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