I ran into a problem today where a graphite iMac was stuck in target mode. Resetting the PRAM did nothing, doing the normal NetBoot shortcut (holding down ‘N’ at startup) did nothing, so I was stuck – for a moment.
To fix this problem, I booted into Open Firmware (using Command+Option+O+F). Then entering these commands allowed the machine to boot from the network:
setenv boot-device enet:10.0.0.2
mac-boot
Of course, substitute “10.0.0.2” for your NetBoot server – if you’ve got one. Thanks to Mike Bombich’s Netbooting Across Subnets page for the help.