It seems to be just one way communication, sound is streamed from the PC to the airport express, and then I can plug speakers in to that. I can't send anything back the other way to skip a track or anything. This would be slightly worse than the current setup of ipod plugged into stereo, or ipod plugged into powered speakers (which we have in the bedroom and kitchen). Maybe I'll get a better solution one day.
Cheers for chipping in though mate.