Although this seems somewhat obvious, the USB port on the Airport Express can be used to charge your USB dongles on the fly. Since I like to put my PowerBook to sleep at night, and the USB ports sleep with it, I like having the option of recharging from the Express. I have tried my Treo and iPod Shuffle to date, and it works beautifully for both of them. One less plug to bring with me when I travel!
[robg adds: It's my understanding that some USB2 ports will charge when sleeping (if they're powered ports), but I'm not sure. I hadn't considered using our Express to charge the Shuffle, so this is a handy tip...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050414213833484