Usually you can just flash a PC card with the OF firmware.
The advantage of OF is it's very flexible and there are OF drivers embedded in OF-compatible cards. Thus the OF version of the ACard ATA disk controller PCI card has an OF driver which can load the kernel and modules from the filesystem. The advantage here is the kernel doesn't need built-in support for that card or some kind of lame disk controller specification in order to boot and root a volume on that bus.
What I would like to know is if the firmware has some kind of VESA driver in case you have an x86-based card which you can't be bothered to flash with the OF ROM. If not how would you be able to get to the OF prompt? (or else you have to pray the firmware doesn't run into problems before loading the kernel)
FYI: Apple's OF technotes
http://bananajr6000.apple.com/OF/technotes.html
