Currently I have a permanent connection using eth0 interface. But when I hook up another computer via ppp dialup using rp3, the defaultroute of eth0 has been removed and replaced by the ppp0 defaultroute. I couldn't find any option to tell wvdial whether to set the defaultroute or not. rp3 don't have this option to tell wvdial either.
This feature will be added to the next version of rp3, which is now in the GNOME CVS tree. The rp3-1.0.4-1 package should also be in our next release of Raw Hide.