One of the concerns is the invasion of privacy.  (Lets be blunt here
folks....you have no privacy now.  When you do to work/the store/
the movies/etc. you're on serveliance TV 300+ times a day.  That
little fact came out in the wake of the London Subway bombing.
Add in the ability to track you cell phone calls, the tracking
of your credit cards/atm cards, the trasnponder for tolls on
yuor car, etc. and you've got essentialy no privacy now.  Add
in that alot of parents in the wake of the child murders/
molestations want to "chip" their kids the same way they
chip their pets and you're privacy is eroded more.

There are cell phones that let you track where your kids are,
get camera images from where your kids, etc.

But then all this won't matter.   20+ - 30+ years from now if
they want to find you, they will enter your name in a computer,
the computer will track down your DNA from every stair rail
you touched, every door you opened, etc.   You won't need
an id card/credit card/etc.  

Since someone just asked me how he could obtain an RFID reader
for the several RFID implanted devices he has (none including
pets or maybe he didn't mention that) and I've spent a few days
reading this stuff maybe I can confuse things even more here.

As one trade magazine put it, there is so much flexibility in
RFID implementation that there is almost no standardization and
no economy from having everyone use the same thing.

RFID can, depending on what they design into it, have a range
from a few centimeters to miles.  The companies are hoping this
is going to be the barcode of the new millenium and be put into
everything from shoes to pill bottles to cars to pets to ups
packages to credit cards to railroad cars.

RFID can use any one of 4-6 different frequency ranges.  And it
can use any number of different chip company's incompatible
protocols, even if using the same frequency.

This was what caused an outcry when one company had been in the
pet RFID business for a while, a second company wanted some of
the business, introduced incompatible RFID devices and they
couldn't be read by the previous reader.  That meant forcing
vets to buy another reader or having a pet not come up with a
code if you checked with the "wrong" reader.

And then there are even battery powered RFID and non-battery
powered (passive) RFID devices.  I'm thinking the battery
powered ones are used for much longer range applications.

Finally there are read-only devices or there are read-write
devices.  For read-only the code is in them, you can read it
out, that is it.  For the are read-write ones they have memory
inside that can be updated, sometimes tens of thousands or even
millions of bits and growing.  I think this is where the
"smart card" people want you to end up going.

However, for pets I believe these are no battery read only
devices and the only variation are the two companies that
are in the business with the incompatible RFID chips/readers,
unless there was enough backlash that they were finally
forced to make compatible parts and/or readers.  But I
believe both of these required a radio signal sent to the
part that then provided the stimulus to get the code back
from inside the part.  I haven't tracked down whether the
two companies were using different frequencies or just
incompatible chips/readers.