Google "mac mini site:ibm.com" for a series of interesting articles with some mention of Open Firmware on the G4 machines. As for intel macs, I don't know how to play with the efi; rEFIt was used to get some bootcamp functionality before Apple released the Bootcamp beta back in the days of Tiger (when I switched to macs).
Okay, now I have my OS X SL Missing Manual open:
Try at boot time:
CMD + V shows unix console messages during startup, logout, and shutdown
CMD + S starts up in single-user (unix command-line) mode
If you are disinclined to google the ibm articles, I believe the command to boot Mac OS in open firmware is 'macboot'; not certain of that.
I have to say that Pogue's book is worth the $22 or so from amazon.com