iptables-1.4.20 update changed soname of libxtables from 9.0.0 to 10.0.0; this triggered a @preserved-libs event for sys-apps/iproute2 and perhaps others. Please consider adding a subslot value to iptables ebuilds the next time a SONAME bump occurs in one of the package's libraries. To support this, the next version bump of iptables should probably also bump EAPI to 5 (or higher if applicable).
1.4.21-r2 has a subslot now http://gitweb.gentoo.org/repo/gentoo.git/commit/?id=6586bd5ff10f807d89b4c6dbaabae7752dc0f1bc