Ran into this internally, as well as a client. Even though I had a 10.8.3 NetBoot image that was diskless, I was not able to unmount the hard drive to image it. Further investigation showed the drive being mounted at /private/var/netboot, not in /Volumes.
Doing some searching, found this Apple Discussions thread:
Joel Bruner's response is the correct one.
Edit /etc/rc.netboot, line:
(i.e. add in the = before the -NETWORK-}
Stop the NetInstall service ,restart it, NetBoot a machine, and voila, you should now see the hard disk mounting properly in /Volumes!
Hope Apple fixes this in 10.8.4. If you're bored, please file a bug report at http://bugreport.apple.com
Hope this helps somebody,