Does OS X have a builtin DHCPv6 client?
Actually yes it does have a DHCPv6 client. I have verified in 10.8.4 but the email thread referenced above talks about it working with Lion too.

To get it to work, you need to set the IPv6 configuration to "automatic" and then on your router, you also need to configure the routing announcements to indicate that hosts should use DHCP to obtain an address. I believe that's what they're referring to in those emails where they say they've got it working with Lion.

Relevant extract from that email (since links may die eventually):


  Quick testing shows that when "Automatic" is used for the IPv6
  setting, OS X will correctly look at the A, M, and O flags of an IPv6
  RA and make use of DHCPv6 when instructed to.


As an example, on a Cisco ASA 5505 running software version 9.1, you'd need something like this:

interface Vlan1
 nameif inside
 security-level 100
 ipv6 address 2001:db8:1234:1::1/64
 ipv6 nd managed-config-flag
 ipv6 nd other-config-flag


The managed-config-flag is the "m" flag and the other-config-flag is the "o" flag from that email. You might or might not need both, depending on what you want to do.

Don't forget that if your DHCP server is on another interface you'll also need something like:

ipv6 dhcprelay server 2001:db8:1234:2::2 dhcpinterfacename
ipv6 dhcprelay enable inside


So yes, DHCPv6 client, but needs a little help from routers on the network. I can't help feeling it's a bit of a mess, but it works.

Note that you'll also get the EUI-64 address and the privacy extension address in addition to the DHCPv6 assigned address. I don't know how to turn them off I'm afraid. In particular, at the moment, I don't know how the OS decides which one to use for outbound connections, which could present some problems.