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
and
 /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)