Quoting Jamie Curtis <jamie(a)pointless.co.nz>:
Once you get nfsroot support into the kernel, you also need to tell it that it's root is an nfs device (the root=/dev/nfs you had) and where to mount the root. So, you need a kernel command line something like the following:
root=/dev/nfs nfsroot=<server ip>:<root filesystem export point> ip=dhcp
The ip=dhcp tells the kernel to get it's IP address via dhcp, not bootp or something else. You will also need to make sure that you have "kernel level autoconfiguration" (or something similar) set in the kernel build as well as compiling your network card driver into the kernel, not as a module (or, you have to muck around with initrd's as well !).
I did a debian install the a firewire drive (difficult as you need to mount the root filesystem before installing the modules needed to access a firewire drive), compiled a kernel with the above options, copied it to the server and booted into Linux. Then i deleted the /etc/mtab file i'd copied to the server, checked the NFSRoot wiki page when i still had a read only filesystem, editted /etc/fstab to set the root filesystem to be rw and here i am, running Mozilla on X. Next step, Debian unstable and kernel 2.6. Thanks for all your help, Jon