I have an existing ethernet network, with a dhcp server, various
equipment and machines, and a firewalled connection to the internet. 
I have a wrt54g (v.3) linksys router, which I would like to run in
"bridging" mode: it creates a wireless network to which wireless
machines may connect, and these machines act as if they were on the
existing physical network.  Specifically, they get their addresses from the
wired dhcp server, and they and the wired machines can easily contact
each other.

Heretofore I have been running the wrt54g in a standard AP mode, in
which it uses its internal dhcp server to assign addresses to the
wireless machines.  This let the wireless machines reach the net and
the wired network machines, but as these machines were are all NAT'ed
out through the wrt54g's address, this made accounting hard and made
it impossible for the wired machines to contact the wireless ones.

From some reading, it looked like the DD-WRT v23 firmware would
support this mode, so I flashed that, but it turns out everyone is
talking about "client bridging" mode, in which the wrt54g is used to
enable a non-wireless piece of equipment (e.g. a tivo) connect to an
existing wireless network.

So: how do I configure the wrt54g v.3 -- with either the official
firmware, DD-WRT v23, or any other available firmware -- to bridge an
existing ethernet LAN to a new wireless network?