The only choice in my m0n0wall router that will work with Comcast IPv6 is to configure the LAN for DHCP-PD.
I also use DHCP-PD, but in my D-Link router that is a separate configuration option from "Stateful" and "Stateless".
If I select the "Stateful DHCPv6" option, my Windows XP and static IP boxes don't get native IPv6 addresses (the dual NIC server boxes do however revert to using Comcast's 6to4 tunnel). -- We can never have enough of nature. We need to witness our own limits transgressed, and some life pasturing freely where we never wander.