I just purchased a 2004 325iC, and am the second owner, received two keys from the dealer. However, one of the key remotes is not working (found this out last night). The owners manual states that the ignition actually recharges the battery in these keys, so I was going to try driving around with the "dead" key for a while to see if that helps. (No, I haven't tried starting the car with the dead key.)
Now that I've seen this thread, I will try reprogramming them first.
Thanks again. This is a great website!