sys-apps/iproute2 (various versions) has dependencies like ">=virtual/os-headers-2.6.27". This worked when os-headers was an old-style virtual provided by some version of linux-headers. But it was just converted to the new-style virtual/os-headers-0, which never satisfies this dependency (the only available version of the new-style virtual is "0"). As iproute2 seems linux-specific it probably makes sense to just replace the virtual/os-headers dependency with a sys-kernel/linux-headers one (the only other package satisfying the new-style virtual is sys-freebsd/freebsd-lib, and it seems unlikely iproute2 would compile against that).
Thanks for reporting, fixed. And no, iproute2 wouldn't work on FreeBSD ;)