i've been backing up wirelessly since 10.5.0 came out...i have a usb drive attached to a headless mac mini, file sharing is turned on, and the mounted drive is shared...a macbook pro, a macbook, and another mac mini all backup to the usb drive wirelessly when connected to the server...i didn't have to do anything special to time machine at all...the macbook pro does leave the house to go to work with me, but once i return home i just connect to the headless mini in the finder and my backups resume shortly thereafter...

all machines are running leopard...the headless mini is NOT running leopard server...