Since getting the Airport Express and going wireless, I've noticed that I get disconnected from the Airport network when I use Fast User Switching to switch over to my iTunes user (I used this MacOSXHints hint to set up a large second shared iTunes library on an external hard disk). After a bit of research I came across this mailing list post. Switching to my iTunes user showed that I could not make any changes to the Network settings. This was when I remembered that I had set the iTunes user to Simple Finder to see what was so simple about it. Switching back to my main user showed that I couldn't make changes to logged in users, so after a quick switch back and logout of the iTunes user, I set it to Full Finder, logged back in, opened up the Airport tab of Network preferences and entered the appropriate network name and password. Note: the password box only appears once you've chosen a network. Voila! No more disconnects when switching users.