If you start up while holding Option-Apple-O-F, can you get back to the Open Firmware prompt?

Another thing to try is a verbose boot - hold down V (or Apple-V) during startup. It will write the startup sequence to screen, and you may be able to see any error messages that come up.

If you have a copy of TechTool Pro or some other bootable system utility disk, it may be worth clearing the PRAM using one of those utilities. DriverGuru (briefly a member here) claimed that they do a deeper and more effective clearout of the PRAM than the usual Apple methods.

Course, it helps if the machine will boot to the CD at all :-/