Hold down c while booting.
Hold alt+apple+o+f down while booting to get into Open Firmware, then:
boot usb0/disk@1:,\\yaboot
(might be usb1 depending on usb port)
It seems only WPA2/TKIP is working (no AES).
agere_sta_fw.bin is in the debian package firmware-linux-nonfree and the file is at: /lib/firmware/agere_sta_fw.bin