If you update from CentOS 6.0 to 6.1/6.2
via Yum, etc:
edit /boot/grub/grub.conf and /etc/fstab by changing
/dev/xvda1 to /dev/xvde1
/dev/xvda2 to /dev/xvde2
To ensure the server can boot.
(You can perform this change via the Console or recovery Console if your server is not booting or getting online)