I have the same problem with a CentOS 5 machine and another user has this issue with RHEL 5. Im wonder what we all have I common.
What kind of storage does your system boot off ?
What is the fstab on youe machine ?
What is the output from uname -a
I was able to clone to the OS disk to a virtualbox virtual machine using dd and that worked first time no need to mkinitrd. So its something todo with kernel drivers and being able to access the / file system.
Regards G