It looks as if the Fedora guys made a decision to update the kernel device detection in RHEL 6.1 when it was being made. I found this out the hard way when I went to build a bunch of new VMs for a customer and get them up to date before deploying them.
You have two choices here:
1) update /etc/fstab and /boot/grub/menu.lst to have the new device name which _will_ be /dev/xvde1 or /dev/xvdeX where X is the device number. This is usually only the case if the Xen 3 config file has the device name as sda1 or sdaX.
2) if you are running Xen and you are using /dev/sdaX as the device name simply change this to be /dev/xvdaX and the Guest kernel won’t have any issues booting / doing kernel upgrades.