It turns out that my post above
wasn't exactly accurate, if not to say incorrect. While you may be able to 'extend' non-Apple WiFi networks ('A'} with an Airport Express ('B') using AirPort Utility, this doesn't necessarily mean that you'll get full functionality.
For instance, with regard to network extension, one way to check which base station you're actually connecting to is to Option-click the Airport icon in your Mac's menu bar. This will then show (in dimmed text) the SSID/MAC of the actual WiFi source below the (checked) name of the extended WiFi base station ('A'). This ID will likely NOT be that of the AE extending the network ('B'). However, such an AE will
work to wirelessly connect printers and hard disks.
That said, it still appears to be possible to do a 'real' and fully functional extension of (certain) non-Apple WiFi networks with AEs, depending on the capabilities of the base stations involved. These capabilities may need to be enhanced with firmware upgrades to support such network extensions. I'll bring up some details in a new thread on the topic (Adding WDS to Linksys WRT300N v1.1
), seeking more information about this.