I recently decided to upgrade my wireless network with an 802.11n access point. Since I had an existing (older) Airport Express, I thought it would be best to get another Apple product. I have the Express plugged into my Internet connection and I have some wired computers in another room that need to have access to it. This is simple to do with a WDS, but the wrinkle in the setup here is that the Airport Extreme really wants to be the main station. Since the Extreme doesn’t support AirTunes, this wasn’t a possibility for me.
After struggling with this for a few hours, I finally discovered the trick: In Airport Utility, when you configure the Extreme, go to the wireless tab and hold the option key while you drop down the “Wireless Mode” combo. This will give you a new option “Participate in a WDS network” and selecting that option gives you the WDS tab where you can set the Extreme to be a remote station instead of the main station.