Essentially you're trying to do the part of the same type of thing that a high availability service would do with failover capabilities.
1. You could build something using ns_update to tell DNS you've changed IP addresses. Subject to the timeout of the DNS record.
2. You could assign a static IP address to the laptop and use send_arp to tell the network which interface that IP address goes with. This change will be pretty quick. In fact you could probably open an ssh connection, switch interfaces, send_arp, and continue using the open connection.
send_arp comes from "fake" and is part of the package you can download at
http://www.vergenet.net/linux/fake/
ethx="whichever interface is live at the moment"
ip_address="whatever static IP assigned to be always up"
macaddr=`ifconfig ${ethx} |awk '/HWaddr/ { print $5 }'|sed -e 's/://g'`
send_arp ${ethx} ${ip_address} ${macaddr} ${ip_address} ffffffffffff
