With Mandriva (which to my knowledge relies on IP links, and hotplug), I achieved this using standard Mandriva GUI tools:
Both eth0 and wlan0 are started on boot. When I connect the ethernet cable, the interface goes up, and route is auto-configured; when I disconnect the cable, route is flushed. Same with wlan0 when I plug/unplug the USB WiFi adapter.
I don't know all the details, but if you want me to look at some config files (eg: in /etc/sysconfig), please ask
Now, the really cool trick would be to bind eth0 and wlan0, so that when both are plugged you have double bandwidth Eg:
my laptop only has USB1.1, so max. 11Mb/s WiFi, and a 10baseT pcmcia card, so max. 10Mb/s. The combination of both could lead to 20Mb/s I guess