[Editor's note: The following note was submitted to me well over a month ago. Somehow, I lost it in the in-box queue, and finally found it this morning as I was cleaning out some old Help Requests. My apologies for the delay in getting this posted! I have no idea if it's still relevant or not, but it seems important enough to merit a mention. Someone please let me know if it's already non-relevant and what the new solution is...]
I use two Beige G3 machines (now upgraded to G4's) and both have had the problem that when I use Startup Disk on System Preferences to go back to OS9, I can't get back to OS X. It appears that the Startup Disk control panel in OS9 never actually writes the correct Open Firmware values to the nvram. Several times I got back to normal by doing a OSX reinstall -- suffering the whole process just to get the nvram fixed.
Today, I found the painfully simple answer -- download System Disk 2.6.2 from Apple's download site and use it instead of the Startup Disk control panel. This worked perfectly...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020126103052337