NFS clients (nfsroot netboot) never purged from showmount -a

I just noticed that our netboot nfsroot implementation never clears its
entry or logs out or whatever when rebooting.  I see the same server in
the showmount -a output over the course of testing various
configurations on various nfsroots:

showmount -a
All mount points on localhost:
sysdev05.phx7.llnw.net:/tftpboot/netboot_rbhave
sysdev05.phx7.llnw.net:/tftpboot/netboot_sysdev05
sysdev06.phx7.llnw.net:/tftpboot/netboot_psunkapur
sysdev06.phx7.llnw.net:/tftpboot/netboot_rbhave
sysdev06.phx7.llnw.net:/tftpboot/netboot_sysdev06


Is there some logout mechanism that we're missing in our nfs client that
would do the right thing here?

sean