ipfw2 keep-state + IPv6 on RELENG_7

Hello,

I'm having problems with ipfw2 + IPv6 keep-state rules, if I use a 
keep-state rule on IPv6 it will only work intermittently (eg. I can 
connect to an FTP site with IPv6 and start to grab a file, but it will 
stall after a few seconds). I am using deny all by default on ipfw, my 
ruleset is as follows (em0 is my external interface):

add check-state

add allow all from any to any via lo0
add allow all from any to any out via em0 keep-state

The keep-state works fine for IPv4 traffic, but IPv6 traffic 
connectivity will only work intermittently with the above ruleset. I am 
running a RELENG_7 cvsuped/built on Tue Oct 16:

FreeBSD akuma.pfak.org 7.0-PRERELEASE FreeBSD 7.0-PRERELEASE #1: Tue Oct 
16 18:30:20 PDT 2007     
peter_at_akuma.pfak.org:/usr/obj/usr/src/sys/AKUMA  i386

Any hints? Is IPv6 + keep-state broken on RELENG_7 or have I missed 
something obvious?

Thank you,

-Peter