Configure it to join your wireless network in client mode:
This feature is only available in the 802.11n Express models, and is not available in the original Express.
If you configure a (summer 2012) simultaneous dual band AirPort Express in client mode:
1) are both the LAN and WAN ethernet ports available or only one of them?
2) If you unplug the ethernet cable momentarily to connect a different device, does the AX drop the wireless connection or is it maintained?