You can, in fact, still boot into single-user mode even when Open Firmware Password is set to on. Problem is, you will ALWAYS have to boot into single-user mode.

From the \"nvram\" man page:
EXAMPLES
example% nvram boot-args=\"-s rd=*hd:10\"

Set the boot-args variable to \"-s rd=*hd:10\". This would
specifiy single user mode with the root device in hard
drive partition 10.

For my setup, I just entered:
nvram boot-args=\"-s\"
and I always boot into single-user mode after going through the Open Firmware password login.

Hrmm... I bet you can do this from within Open Firmware itself (cmd-option-O-F), if you can get the syntax right; OF's command parser is really ugly. e.g., set boot-args 's' said unknown word: set, but boot-args 's' said unknown word: s. shrug, i'll cruise the kbase I guess.

sudo nvram boot-args=-s

setenv boot-args -s