Hi - I've done this with my Squeezebox Radio, where I have connected an Airport Express to Line In on the Squeezebox. It works well, I get the best of both worlds - Airplay when I want it, but can just walk up and play Internet Radio or Pandora without having to hunt for the iPhone/iPad or chew battery. Only nitpick is that I have to select "Line In" on the Squeezebox to switch from radio back to Line In for the Airplay - it auto-detects when you plug in a cable, but since Airport Express stays plugged need to manually select the input. I have not tried on a Squeezebox Touch but should work as well, probably better since SB Touch supports digital which I believe Airport Express can output with the right cable.
Good luck!
Full disclosure - I do work for Logitech.
