It acts an an AP on non Airport router networks just fine. I've ran the following in that mode with various other routers (linux box, Linksys POS, Buffalo)

Airport Graphite (released in 99)
2x Airport Express (802.11g) (the solution I had when you were in CO)
Airport Express (802.11n)
Time Capsule

Attached is a screenshot showing the setting to put it into plain access point (bridge) mode.

I still have a Linksys 610n still, acting as a gigabit switch in the middle of my network. I'll try turning on its wireless and configuring it for WPA2. I'll then set up one of my express units with WEP to see if I can recreate the 3GS issue.

As for buying a bunch of Airports for your work, I do think it may be overkill, since as you said, they have a ton of features that would not be worthwhile in your situation.