As of OSX 10.5.8, there seems to be a problem with static assigned IPv6 Addresses. You can fill in the dialog, but it does not actually configure the adapter.

Work around:
ifconfig en0 inet6 2001:48:24:12::12/64
route add -inet6 -prefixlen 0 default 2001:48:24:12::1

Note: this is not actually my IP (v6) address