I messed up my copy of /etc/fstab and when I booted into obsd 3.1, it threw me into single user mode. /usr was not mounted with editors and /etc was mounted as read-only. Here's how I fixed it.

mount -w /dev/wd0a /
mount /dev/wd0g /usr
export TERM=vt220

vi /etc/fstab
hit x for the characters to delete
hit i to start inserting text
hit esc to escape from insert mode

type :wq to save and exit
reboot