Just FWIW:

I've now got the ELF image of the ofwboot program (thanks to Tsubai Masanari)
,  and booted it over the net.  It gives me the NetBSD boot banner and then
obviously tries to make a bootp request. tcpdump shows some packets being sent,
but they are completely garbage, that is, all bits (even in the ethernet
header) are set to one. After a while, ofwboot displays an error message and
gives me the Boot: prompt.

Then I tried to boot a kernel from the boot floppy image I burned onto a CDROM,
by typing "cd:0" at ofwboot's "Boot:" prompt. It apparently found the kernel
and loaded it (printed something about text,data,bss sizes and start address),
then again froze the machine.

Maybe the iMac's OpenFirmware is still not good enough for netbooting? Or could
it be a quirk in ofwboot?

Best regards,
Sebastian Lederer