I understand the basics of passive RFID tags but one answer that has
failed be has to do with range. Various manufactures of RFID tags and
readers talk of the different frequencies that they operate with and
tie that into their discussions  of range (distance between the tag
and the reader), but what I fail to see is why the power of the signal
broadcast by the reader does not also correlate to the range
attainable.

My specific example: Our metro system uses RFID 'SmartPass' cards for
storing fare values. Normal usage has you waving your card within a
few inches of the turnstile to allow passage. I would assume that the
reader is sending a fairly weak signal so not to affect the other
turnstiles and cards in the area.

What prevents a RFID reader from being crafted that sends a signal on
the same frequency but at a stronger power (Watts?) and reading all
the cards carried by everyone in the station? Questions arising from
dealing with multiple simultaneous card responses aside.

The tags are powered by the reader so the transmitted power would have
to be considerable, which would fall outside the FCC guidelines for
that particular frequency.  Also, the reader would have to have a much
more sensitive receiver (since the tags have a very low output) which
makes it more susceptible to noise (unreliable operation).  Hope this
helps.

Yea, mostly has to do with the FCC.

Reading multiple cards?  Not a problem.

Note many stores use RFID with regards to loss prevention/theft. 
Those little sticky tags that attach to your (insert stealable,
expensive but small product here, like a CD) work from about 5 or so
feet away.

Many tollbooths also have RFID technology which allows the receiver to
be activated from 5-15 feet away.

So, technically, you could have the same thing in your wallet, though
the receiver would have to be bigger and would no longer be
comfortable or easy to carry.

Not impossible, just more expensive and probably not worth it.

Also, what if somebody in the station didn't have a card?  Nobody
would be the wiser, and that person would get a free ride...but if
they have to hold it a few inches away from a transmitter specifically
to unlock the turnstile, then it keeps people a bit more honest.

My question is actually geared toward the privacy implications of RFID
range rahter then building a practical application.

I agree the technology could have boosted transmiter power and read
some cards repeatedly at distances of several meters. At still more
power you would be exposing the closest people to ilegal levels of RF
radiation which might damage their health. The receiver(s) would need
to be designed for many different frequencies and codes to read cards
that people used for a wide variety of purposes. It would be very
costly, unless mass produced. Perhaps you will soon be able buy for
$99 at Walmart a device that will read some of the RFID cards in
wallets and purses when an appropriate transmiter is nearby.   Neil

Yes, there are quite a lot of privacy issues concerning RFID.  After
all, what's to stop someone from taking a handheld transmitter around
to purses and wallets, silently copying the code from each RFID card?

For the purposes of this comment, let's be clear that the transmitter
is like those poles you walk through at the supermarket, and the
receiver is the actual card.

Anyways, we use RFID at our work to gain entry into the building. 
But, if we look at how these guys work, which is basically that the
transmitter is always transmitting, the card is powered from the
transmitter (E = hv), and reflects a signal with that card's
identifying information.  Well, it seems like an easy step from there
to store the information, put it in another device, and send it out
for the base station to pick up...