In an example, we did a yum update that updated the Fedora kernel from 2.6.18-1.2239.fc5smp to 2.6.19-1.2288.2.1.fc5smp. On the subsequent reboot, there was a kernel panic on bootup because the ramdisk image could not find the lvm info in /boot/initrd-2.6.19-1.2288.2.1.fc5smp.img (possibly because of a bug with this kernel). This is a common problem and this manual explains how to fix it.