AirPort Express Compatibility? (by Jeremy Zawodny)
Sonoma County Airport Express Inc.
Apple AirPort Express Base Station (802.11n) review: Apple AirPort Express Base Station (802.11n)
Since the Apple Express Assistant will only automatically detect Apple wireless networks, you have to install the device initially using an Ethernet cable from your router as its own network.
Handy as then you can attach more storage.
There are still some oddities, but it works most of the time.