Airport Express Technology Overview
This Apple white paper in the .pdf format provides technical details on various
aspects of the Airport Express.
http://images.apple.com/airportexpress/pdfs/L304895A_APxps_TO.pdf
In the document, Apple reports this about the RJ-45 port on the Airport Express:
Ethernet
The WAN port connects to a DSL or cable modem or local Ethernet cable to provide high-speed Internet access to the users on your wireless network. When the base station is being used to bridge your network, you can use the Ethernet port as a local area network (LAN) port as well.
I believe the built-in auto sensing helps achieve this. So, as I interpret this paragraph: the AEX RJ-45 Ethernet port should be able to function as either a WAN port to connect a cable or DSL modem or a LAN port to connect to other network devices, such as a printer or (I'm making an assumption and an interpretive leap of faith on this second one), an Ethernet hub or switch...Would anyone agree with this interpretation?