While virtualizing a RedHat RHEL 6.2 machine, I've made a pleasent discovery: after cloning the disk, the VM booted without any initrd rebuild.
The source was an HP BL460 G7 and the target VM was created with the VSphere wizard using a typical Red Hat 6 x64 configuration: version 7 vm, paravirtual scsi, vmxnet3 networking.
The VMDK was created as big as the source physical disk.
I've followed the procedure for RedHat 5 P2V, with one notable exception: the internal disk on RedHat 6 is called /dev/sda even if it's really a SmartArray RAID drive.
After the ssh command:
# ssh root@P "cat /dev/sda | gzip -1 | cat" | gzip -d >/dev/sda
I've simply rebooted the VM and it came up on the first try.