Hey guys, I just set up Debian Lenny to be my IPv6 router. I noticed that there really aren't any docs on this specifically, so I wrote a nice little post on my blog:http://wanners.net:8000/blog/2011/01/debian-lenny-ipv6-gateway/
I'm currently messing with DNS, so if that doesn't work, here's an alternate (IPv4 only) link:http://wannerhome.dyndns.org:8000/blog/2011/01/debian-lenny-ipv6-gateway/
Hope this helps some poor soul out. It would have really helped me, I know.
If anyone who knows more about this stuff wants to offer any input, feel free to post a reply here or use the comment form there.
Update: This works exactly the same way in Squeeze and kept working with no changes when I upgraded from Lenny.