For Redhat 7.0 and 7.1, you should not need to modify any initscripts to enable LVM at boot time.
For versions of Redhat older than 7.0, it is necessary to edit the file /etc/rc.d/rc.sysinit. Look for the line that says "Mount all other filesystems" and insert the vgscan and vgchange commands just before it. You should be sure that your root file system is mounted read/write before you run the LVM commands.
You may also want to edit the the file /etc/rc.d/init.d/halt to deactivate the volume groups at shutdown. Insert the
vgchange -an |