SCENARIO:
We wanted to connect two small business offices with a VPN.  Our existing firewalls were Linksys WRT54Gs.  We chose to run the DD-WRT VPN firmware and utilize OpenVPN to help resolve our need.  (These instructions should also work on the DD-WRT support models like the Allnet ALL0277, Buffalo WHR-G54S, Buffalo WHR-HP-G54S, ASUS WL500G-Deluxe, Motorola WR850G, Siemens  Gigaset SE505, Ravo W54-R, and Askey RT210W.)  

OpenVPN can be run in two modes: routed and bridged. The steps below set up a routed VPN so we could keep our existing subnets at each site.  If you are looking for bridged VPN instructions client here. One of the WRTs should be the VPN server and the other shouild be the VPN client.  This senerio works well when one site has a static internet IP address and a valid DNS entry while the other site is setup with DHCP.  If both sites are setup with DHCP internet addresses, the server VPN should have a Dynamic DNS entry at a provider like www.dyndns.com.  DynDNS.com is a free service and there is a client built into DD-WRT.   

These instructions are written assuming that you will configure the VPN server WRT first and the client WRT second.

STEPS:
1) Install the v.23 dd-wrt VPN final or beta version of firmware available HERE -- Instructions for installing DD-WRT are available HERE

2) Logon to the web management interface in DD-WRT.  Select the Administration tab.  Scoll down until you find the JFFS2 Support information.  JFFS2 must be enabled.  If you have never enabed JFFS2 before you will also need to select the Clean JFFS2 enable button to initialize the file system.  Scroll to the bottom and select Save Settings. 

3) Select the Administration tab and then the Services subtab.  Scroll down to the OpenVPN client section and make sure that Start OpenVPN  is set to Disable.  If you had to disable it, make sure you scroll to the bottom and click Save Settings.  You have to leave this disable because the configuration in DD-WRT is design to act as a client to a vPN server running on a full-blown linux or other host.