I think everyone missed an important detail from the initial question.  
He stated he has already tried "spoofing" the MAC address.  I was not 
aware m0n0wall had this functionality, but assuming it does, and he is 
using the MAC address from his working router, that should not be the 
problem.

Jonathan Merrill wrote:

>Team -
>
>Recently, I have discovered that Adelphia is now doing MAC authentication
>for their cable modem residential customers.  This means that you have to
>tell what Adelphia what network card and MAC will be used to allow a DHCP
>lease to be given.  This behavior also happens if your IP address changes
>from a routable to a non-routable (i.e,. 192.168.x.x).
>
>This same scenario happened here in Ohio.  How it was solved was by calling
>Adelphia Tech Support, give them your MAC, and reset the cable modem.  Up
>and running in 2 minutes.
>
>Hope this helps.
>  
>