i encounter this bahaviour when a default route already exist. The
following patch should correct the problem (at least, it works for
me):

Index: usr.sbin/dhcp/clientscript/dhclient-script
===================================================================
RCS file: /data/netbsd/cvsroot/src/usr.sbin/dhcp/clientscript/dhclient-script,v
retrieving revision 1.11
diff -b -u -r1.11 dhclient-script
--- usr.sbin/dhcp/clientscript/dhclient-script  23 Sep 2006 22:54:43
-0000      1.11
+++ usr.sbin/dhcp/clientscript/dhclient-script  29 Sep 2006 19:46:34 -0000
@@ -59,6 +59,9 @@
 add_new_routes() {
        for router in $new_routers; do
                route add default $router
+               if [ $? -gt 0 ]; then
+                       route change default $router
+               fi
        done >/dev/null 2>&1

        set -- $new_static_routes