Configuring a PPP Half Bridge/IP Passthru

[Andrew Watts <systemstalker@gmail.com>]

Hi,

Im trying to create a PPP Half-Bridge/ZIPB/IP-Passthru so that I can
pass through my public IP address to a device connected to the
ethernet interface of a device that uses a 3G modem for connectivity.
This product exists but I want to do it with my own hardware.  I have
an NSLU2 that I have connected to a 3G cellular into the USB port, I
then want to take the IP address from the resulting PPP and give it to
an embedded device so that it has a publicly routable IP address that
can be accessed directly.  Cradlepoint has such a device, I want to do
the exact same thing just with Linux :).  The link below explains what
I am try to do as well including the product in question.  I cant
bridge a PPP interface to an Ethernet interface, if I had to do that
where would I start to make modifications to the kernel?

http://store.drwireless.com/crceippabrad.html

Anyone have any Idea how I might do that?