I'm doing exactly what you want.

Turn off 'Distribute IP addresses' on the
base station. Turn off any dhcp/nat
on the basestation. Turn on ethernet/airport bridging. You just want
the base station to be a dumb bridge (from
a TCP/IP point of view).

if the base station itself cannot get a
dhcp address from the router, then
give it a static address on the same
subnet as the router and set the routers
dhcp range so that cannot assign that one
address.

for example, statically give the airport
192.168.124.2 and tell the router to use
192.168.124.3 onwards toward 255 (w/
subnet of 255.255.255.0)