I have had exactly the same problem as Molnar Gyula.

I have been trying to install NetBSD 3.1 from a CD onto a Blue & White G3 (OFW
3).  At first all went exactly according to INSTALL.html; the installation
kernel booted correctly, found my pdisk partitions, installed everything to
wd0a, put the working kernel "netbsd" in the root of /dev/wd0a, and so on.

I then reboot from the OF prompt with 

   boot hd:4,\ofwboot.xcf hd:5,/netbsd

and ofwboot.xcf finds the working kernel "netbsd" and starts to boot it.  It
gets through almost all of the dmesg messages until 

   Boot device: <unknown>
   Root device:

and there it freezes.  The keyboard doesn't work at all, and I have to press the
power button on the front of the G3 case to start again.

I don't think it's an Open Firmware problem because netbsd does start to boot -
it's obviously in the expected place and accessible.  My best guess is that it's
a kernel problem, and that something is going wrong with the way the kernel
autodetects the root device.  

There is a reference to something similar in the NetBSD/macppc FAQ, under
"General Problems & Questions", article "Why does the kernel always prompt me
for the root device?".  Here it says that sometimes " ... the kernel can't seem
to find the root device."  Perhaps we have a version of the same problem, except
that the kernel freezes completely instead of just giving up and asking the user?

The FAQ article recommends rebuilding the kernel with the config file entry
altered from 

   netbsd root on ? type ?

to 

   netbsd root on wd0a type ffs

which will simply bypass the autodetection.

I can't recompile the kernel on the G3 because BSD won't install ... but I also
have NetBSD 3.1 for i386 installed on a 586 computer, and I could use this. 
Should I download the macppc source onto the 586, edit the config file,
recompile, and use this as my working kernel on the G3?  Has anybody tried this,
and is freezing at "Root device:" a common problem?

I've only been experimenting with NetBSD for a month, and this is my first post
here, but already I've found this site really useful.  Thanks!

Nick Sheppard