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).