I have that exact setup.

Get a wireless access point ( I use
a Lucent RG1000 cause it was free) and plug it into one
of the router ports. Configure the
WAP to do nothing more than 'bridge'.
You do not want it to do any DHCP/NAT
or much else (turn on security like WEP,
MAC address restriction and closed network
if you can).

The router will assign DHCP'd addresses
to both the WAP and the powerbook.

(I will say, on the Lucent I have, that
didn't work. I needed to manually assign
an IP address on the WAP on the same DHCP subnet
the router uses. No big deal). The
powerbooks and PC laptop get everything
they need from the router via the WAP.