Grml 2013.01-rc1 can't be PXE booted by default because due to a change in the
kernel some nfs code is now available in separate modules nfsv2, nfsv3 and
nfsv4. Those modules are missing in the initramfs and therefore network boot
fails.
I provided a fix for initramfs-tools:
http://anonscm.debian.org/gitweb/?p=kernel/initramfs-
tools.git;a=commit;h=3b2cca35d3a02448cb56ebac8bb0ced4f6654746
and released it as initramfs-tools 0.109-1+grml.1 which was uploaded to grml-
testing repository (until we've decided in i-t upstream how to proceed with the
fix WRT wheezy/unstable/experimental).