Does kppp actually set the default route? After you dial the isp type route and post the results.
Code:
$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth1
192.168.0.0 * 255.255.255.0 U 0 0 0 eth0
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default <isp-gateway> 0.0.0.0 UG 0 0 0 ppp0
It may be that you have 2 default's, in that case you'll need to edit your ifup-ppp script to remove the eth0 default, and edit ifdown-ppp script to add the eth0 default.