network config with two network cards as route to gateway

[david at thekramers.net (David Kramer)]

So I got SuSE 9.1 (Ugh.  Don't ask)  installed on the new Thinkpad T42.  It 
has a built-in 10/100base-t ethernet, and a built-in minipci 802.11x card. 
I'm still struggling getting the Centrino wireless card (IBM 2200) working 
right, but at one point I had it working (open to suggestions on this too. 
Fooled with linuxant's driverloader before I saw you have to pay for it).

So here's the thing.  Sometimes I'm going to be connected via eth0 (hardline), 
and sometimes I'm going to be connected via wlan0 (wireless).  Even when I got 
the wireless network card on speaking terms with iwconfig and ifconfig, it 
still wouldn't pass any data.  I am *GUESSING* that the problem might be the 
routing table.

So how do I set up the routing table such that when eth0 is live, I use that 
to connect to the gateway, and when wlan0 is live, I use that to connect to 
the gateway?  Do I need to set up a script to change the routing table each 
time?  Or can the routing table be smart enough to use wlan0 when eth0 is down?