If Command-S does not work to boot into single user mode, you probably need to reset OpenFirmware (Command-Option-P-R before the sound chimes at boot time).
When I recently had to do this on an Ultra 10 that was in full-security mode (preventing me from doing anything unless I entered the right password), here are the steps I followed. #Turn the Power off #Remove the NVRAM module (by the pci riser card) #Turn the power on (wait for it to complain about a bad/missing nvram and then either give you the 'ok' prompt or start scrolling screens full of error messages) #Insert the NVRAM module, while the power is still on #Follow the instructions at the 'Reprogramming the NVRAM module' link above to program a HostID and Ethernet Address for the machine