I found a solution to charging my iPhone while my Mac Mini is sleeping overnight. I didn't want to leave my computer on all night just to charge my iPhone, and didn't want to switch between the computer and wall charger every day. Usually, when you put your Mac to sleep, it cuts power to the USB ports, so your iPhone won't charge. To solve this, I ordered the "iPod Dock Connector to Firewire and USB 2.0" from the Apple store. With this cable, you can use the USB end to sync with your computer, and use the Firewire end to charge the phone overnight. Both ends stay connected to their respective ports all the time. This works because the Firewire ports still provide power when your Mac is sleeping, unlike USB. I currently have it plugged into a powered Firewire hub, but I've also tested it with a Apple 23" display and MacBook Pro.
[kirkmc adds: I can't test this, not having an iPhone here in France, but it makes sense that just plugging the phone into a powered hub will charge it; does it charge as well from a powered USB hub? Can anyone confirm?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070928085653963