Are you running Linux on that laptop? If yes, then the problem might be solved by the fact that it will answer (by default) for ARP queries for all IPs on all interfaces. For example, if you have IP1 on interface 1 and IP2 on interface 2, the machine will respond to an ARP query for IP1 arriving on interface 2 (and will answer with interface 2's MAC address).
You'll still have a problem with keeping the interfaces up, I guess. Are you taking the ethernet interface down when you disconnect the cable?
