(E)NBD as root file system

I have read (elsewhere) that NBD as root file system is possible through 
custom modifications to initrd. I presume this approach requires the 
creation of a ramdisk from which to run the NBD client. Once the NBD 
client is running and attached to the remote NBD server, there must be 
some mechanism by which this remote file system can be made the root file 
system.

Does anyone have a recipe to make the above happen for a Linux 2.6 kernel?

... Dave