I'm not sure if that means that computers on a wired network using the same ip address range as the wireless one actually can't stream music to the AE, or whether it's just the way they've phrased it (assuming that all your clients will be wireless). Might be worth a quick call to Apple support to clarify that?
Thanks, but on my laptop (now wired), I have the AirPlay icon and can select "Living Room" (which is what I named the Airport Express speakers).