I had the same issue with my netboots hosted on a 10.8 server. I followed these steps:
http://www.macos.utah.edu/documentation/administration/setup_netboot_service_on_mac_os_x_10.6.x_client.x_client/setup_netboot_service_on_mac_os_x_10.6.x_client-diskless_netboot.html
and now my 10.5 10.6 10.7 and 10.8 netboots all hosted on 10.8.1 run diskless.
The "enable" and "diskless" options were disabled by the Server 2.1.0 updater...er...so was DNS. I had to spend some time configuring things again.
Like iOS6 Maps, the Server 2.1.0 update wasn't without issues.
Steve Jobs must be rolling over in his grave.
Don
I did what andy did simply for performance issues and it turned out great.
Creates a local ramdisk and uses that for the shadow file so it isn't hosted on the network and it allows "diskless" operation allowing the internal HD to be unmounted
http://www.macos.utah.edu/documentation/administration/setup_netboot_service_on_mac_os_x_10.6.x_client.x_client/setup_netboot_service_on_mac_os_x_10.6.x_client-diskless_netboot.html
@andy
your suggestion fixed the issue I'm having. Thank you!
I had this problem, ended up being I added the NetBootClients0 group to have Read/Write permissions on /Library/Netboot/NetBootClients0 and propagated the ACL permissions. Which is the same effect of setting Everyone to Read/Write UNIX-wise, since the group NetBootClients0 includes Everyone. If you are on 10.6 the NetBootClients0 group doesn't exist so you could just change the Unix permissions, but I believe this is a 10.7/10.8 server issue, so that's kinda moot.
Anyway, that's what it was, the netboot client didn't have permissions to write to the share so if fell back to the local storage. Might have something to do with using HTTP vs. NFS I wonder?
Anyway, that did it. Using the above script will work because you are using RAM as a disk, but it's not solving the underlying problem of permissions, which this seems to be.
So even though I cleaned up the permissions, it seems the server creates Shadow files with incorrect group permissions that inhibit proper diskless net booting. Only after a couple client reboots are the Shadow file perms corrected (rw-------)
I've filed a bug with Apple, mirrored at Open Radar http://openradar.appspot.com/13937874
If others have this issue, it might be worth it to file bug report (bugreport.apple.com) so this is fixed (maybe in 10.9 but better than never!)