You can configure most DD-WRT capable routers as a simple 4 or 5 port switch. A switch is similar to a hub in that all devices connected to it will be in the same broadcast domain and can communicate freely with each other.
The configuration is nearly identical to that of a Wireless Access Point. The only real difference is that we will be disabling the radio. One of the main benefits of using a Switch over a WAP is that the throughput between wired machines (100-1000mbps) is usually far greater than wireless (~22mbps actual for 802.11g).
Connect to the router @ http://192.168.1.1 using a wired client
Note: If this router is wired to another router, there may be conflicts (both routers could have the same IP address). For the time being, disconnect this router from the main one or create a static ARP entry.
Open the Setup -> Basic Setup tab
WAN Connection Type : Disabled
Local IP Address : 192.168.1.2 (i.e. different from primary router and out of DHCP pool)
Subnet Mask : 255.255.255.0 (i.e. same as primary router)
DHCP Server : Disable (also uncheck DNSmasq options)
(Optional) Gateway/Local DNS : IP address of primary router
(Optional) Assign WAN Port to Switch : Enable this if you want to use WAN port as a switch port
(Optional) Wireless Network Name (SSID) : "Custom"
(Optional) Wireless SSID Broadcast : Disable
Open the Services -> Services tab
(Optional) DNSMasq : Disable
(Optional) ttraff Daemon : Disable
Open the Security -> Firewall tab
(Optional) Uncheck all checkboxes except Filter Multicast, then set SPI Firewall to Disable
Open the Administration -> Management tab
(Recommended) Info Site Password Protection : Enable
(Optional) Info Site MAC Masking : Disable
(Optional) Cron : Disable
(Optional) 802.1x : Disable
(Optional) Routing : Disable
Apply Settings and connect ethernet cable to main router via LAN-to-LAN uplink*
You may wish to set up some WPA passphrase on the Wireless SSID, just in case the radio was to turn on briefly or re-enable itself for some odd reason. You can also navigate to Advanced Wireless settings and lower the TX power to 5mW or so. It may help keep your hardware temps cooler.
To connect the switch to the main router, you can probably use either a patch cable, straight-thru, or a crossover cable. As far as I know, most dd-wrt capable devices can do autosensing (at least mine do) so the cable type doesn't really matter.
You can connect the switch to the main router via LAN-to-WAN so long as you have assigned the WAN port to switch (see step 3). Although some problems have been reported when doing so...maybe not all routers support it?
If you are configuring for wireless-N in any way (mixed or only) you must use AES security.