On Fri, 01 Sep 2006 09:40:09 -0400
  Mark Opert <marko at clinedavis dot com> wrote:
> How can you allow DNS traffic, or any other traffic, to pass through 
>the
> m0n0wall before the user authenticates on the portal page?

Under the captive portal settings look for "Allowed IP addresses" tab, 
and add your IPs.  Not you cal allow an internal IP out to the world, 
or an external IP available to all.  This can be called "Walled 
Garden" on some boxes.