I had to boot one of our iMacs into Single-User mode using the Command-S during start-up. The problem is that the (ahem) Mac wouldn't recognize the keyboard (a Mac keyboard no less) until the system booted.
Resolution: Dell USB keyboard. At the login prompt, I told the Mac what kind of keyboard I had (it's a wizrd that came up automatically) then rebooted. Instead of COMMAND, though, I had to hold the WINDOWS key. Make a note of that. So, boot with WindowsKey-S and that will take you to single-user mode. Hope this helps someone.