I think the iMac will net boot.  In fact when I tried linux kernels
(didn't have a netbsd one at the time) it gave the exact same results you
were saying the cd gave you.  Double check your bootp server.  We screwed
up the fist time setting it up and spent a lot of time to discover what
was a typo.  Our bootptab looks like this

imac:bf=/tftpboot/g3installer.coff:dn=dogcow.org:ds=10.0.1.4:gw=10.0.1.4:ha=00.0
5.02.D3.CC.F7:ip=10.0.1.22:sm=255.255.255.0:ht=ether:bs=auto:

We also hav the tftp server setup on the same machine.  In this instance
we are trying to load a coff kernel.  I can't recall what the difference
was but the iMAc did react differently to coff kernels than it did to elf
kernels.  The OF device tree pretty much shows what the OF will know about
i.e.  iso-9660 filesystems etc.

Here is a web page we made about the iMac Open Firmware that has most of
the device tree in it.
http://azrael.uoregon.edu/imac

wes

~~~~wesman@gladstone.uoregon.edu~~~~~~~~~~NeXTMail OK!~~~~~~~~~~~~~~~~
Vax  a vicious creature known to eat 110AC and quotes through its 
*DCL*.  Vax are usually found in groups of Vaxen called clusters where 
they lay in wait to ravage thier prey known as users.

On Thu, 3 Dec 1998, Sebastian Lederer wrote:

> Date: Thu, 03 Dec 1998 09:51:25 +0100
> From: Sebastian Lederer <sleder2s@smail.inf.fh-rhein-sieg.de>
> To: port-macppc@netbsd.org
> Subject: Re: Netbooting the iMac
> 
> 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
> 
>