I bought a used AirPort Express today and I can’t believe it took me this long to figure this out. AirTunes is a pretty phenomenal concept, but getting the whole set up to run with my WRT54G was painful.
It was unable to run in client mode. I tried a couple of times to get it going through the automatic setup to have it only do AirTunes, but the yellow light kept blinking. In the end, I set up with in WDS mode, which also turned out to be difficult to use. The trick was making sure both the WRT54G and the AirPort Express had the same wireless channel, and when putting the WEP/WPA key in the AirPort Express to prefix it with a “$”. Who knew?
In any case, it’s up and running and amazing. I highly suggest also throwing Airfoil in the mix so you can route YouTube videos and sites like Pandora through AirTunes.