Brett Wickens reports "Since applying the G5 Firmware Update, my G5 will no longer boot from my Diskwarrior CD boot disk (which contains OS X 10.2.3). I get a brief flashing folder icon, then the G5 continues booting from the internal drive. Doesn't matter whether I select the CD in Startup Disk, hold down the C key at startup, or hold down the option key. Interestingly, the CD shows up as a bootable volume in Startup Disk, but not when I hold down the option key at startup."
Note that Alsoft's documentation indicates that currently shipping DiskWarrior CDs are not bootable on any G5s, nor any other Macs released after September, 2003 (though users have had varied success).
However, you can successfully run a DiskWarrior rebuild process if you've got an extra Mac that supports FireWire target disk mode, as well as a spare FireWire cable (6-pin to 6-pin).
Simply designate the G5 as the target disk (hold down T during startup), and it will appear as a normal FireWire volume on the host computer, which can be used as a valid, non-startup volume with DiskWarrior.