Changing net devices & routing problem

[Steve Snyder]

I want to switch on the fly between several network interfaces: eth0, 
wlan0, and ppp0.  It is easy enough to take down one interface and bring 
up another.  The problem is that the routing table is not set correctly 
for the new interface.

If I specify "GATEWAY=192.168.0.1" in /etc/sysconfig/network then traffic 
is routed correctly for eth0 (my usual interface).  However that is 
innappropiate for either wlan0 or ppp0, and no packets are routed with 
this config in place.  If I do not specify the gateway, then it is eth0 
that fails to route packets correctly.

How can I specify the default route on a per-interface basis?

Thanks.