Attached is a simple patch against 3.8.0-5. With this patch, the install completes and config files are installed, but of course ddclient won't function without the user editing the configuration files appropriately.
Patch is against -5 because -6 apparently has issues with postinst; submitted as bug 555151. I imagine the same workaround would function there.
diff -u -r ddclient_3.8.0.orig/debian/postinst ddclient_3.8.0/debian/postinst --- ddclient_3.8.0.orig/debian/postinst 2009-11-08 13:08:31.000000000 -0500 +++ ddclient_3.8.0/debian/postinst 2009-11-08 13:09:06.000000000 -0500 @@ -53,7 +53,7 @@ # if the hostslist was blank, let the user know some possible reasons if [ -z $hostslist ]; then - db_input high ddclient/blankhostslist + db_input high ddclient/blankhostslist | true db_go fi }