The first (that I know of) NAT64 implementation was in OpenBSD 5.1
release and there's support in the default firewall/packet filter (pf). I
think FreeBSD has this code working as well, but I'm not sure, and I'm too
lazy to check.
I find it ironic that the h-online was 'corrected' following a reader's comment ("IPv4 host can not initiate a connection with a IPv6 only host with NAT64"). This is actually false. The NAT64 gateway _can_ do this, provided you have configured it correctly.
I found this http://home.nuug.no/~peter/pf/eurobsdcon2012/nat64.html quite helpful, as well as a post on the mailing list [1] describing commong scenarios. Of course, the pf.conf manpage describes with all the details, but that should go without saying :-)
[1] http://www.mail-archive.com/misc@openbsd.org/msg107255.html