Sounds like something got weird in openfirmware.
I'd boot into a rescue shell from an install disk, and at the second yaboot boot: prompt, hit -tab- to stop the countdown, and type:
(or whatever rescue option is shown, ie rescue-powerpc64 for G5's) and then get to a shell on your root partition. For default installs where guided partitioning did it all automatically for you, it will probably end up being /dev/sda3 or /dev/hda3 etc.
Then as long as your /etc/yaboot.conf file is ok, redo ybin
In the rescue mode, you are already operating as root, so no need for sudo. Exit the shell (just type exit and reboot.
ybin -v -C /etc/yaboot.conf
Ideally it should no longer go into a loop..