I've actually designed a system with these chips ("RF ID tags") in
them, albeit not for cars. Allow me to bore you with some details:
  :)

The cheaper ones do not have batteries in them. They are activated by
being in close proximiy to the transmitter (somewhere near your lock, I
suspect) and get a small charge from the transmitter's RF field, just
enough to burst a serial number to the transmitter.
One can have tags with a preset serial number but some also contain
data that is one time programmable from the factory. The more advanced
ones are encrypted.
On my Altima 2K it seems to just burst a string of binary data.

The more expensive tags have a battery in them and do have some memory.
I find it hard to believe they'd use that inside a car because there
really isn't much of a need to write any information to them.
I've seen the dealer pair my key with my car by plugging a special tool
in the diagnostics port and pressing a sequence on the tool while my
new key was inserted. One would imagine the computer just takes note of
the key's serial number as being legitimate. So most likely the
programming lost is that of the car's computer.

I do know that my car had issues when another RF ID was placed next to
the key. It being RF (and RF being absorbed easily by any metal in the
viscinity), it could be that the other keys on they keychain interfere
with the communications between the altima key and computer and one
needs to use the key all by itself. Try it to see if that helps.
Not sure why they put this tag on a key -- seems to me that's just
another thing that can easily fail but maybe that is just what they
want, huh?

Remco