Kickstart

Issue
The 'clearpart --initlabel' option in a kickstart no longer initializes drives in RHEL 6.3.

Environment
Red Hat Enterprise Linux 6.3
Anaconda (kickstart)

Resolution
Use the 'zerombr' option in the kickstart to initialize disks and create a new partition table.
Use the 'ignoredisk' option in the kickstart to limit which disks are initialized by the 'zerombr' option. The following example will limit initialization to the 'vda' disk only:
zerombr
ignoredisk --only-use=vda

I found the solution in this forum post while trying to find out why my kickstart install is prompting with a warning while initializing a blank drive.