At least in the case of my card, I had told (then) Citibank to close the account when I stopped using it. But they still sent me checks for the account and a replacement card from time to time. It seemed like having an extra card available might be handy under some circumstances, so I didn't press the point.
My husband opened the brochures with the new terms on our dormant accounts this morning. The current APR on those account was (until we closed them this morning) apparently 7.99%, but we were so shocked by a possible APR of 29.99% that we didn't want to leave such a potentially destructive financial instrument lying around. It is a bit of a shock that there exist circumstances under which credit card companies can legally charge that much interest.
We cancelled both cards. Bang.
This post on a personal finance blog described a simple scenario under which one could find oneself paying that kind of rate.