I have found there is a bug in GRUB2 that causes a server fail to boot if you create a lvm snapshot.
https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/563895I have confirmed this is true for Zentyal2 and ubuntu 10.04, and i think it is solved in ubuntu 10.10, but not sure at all.
I have found this by creating a root snapshot. Removing the sanapshot lv from a live CD makes the server boot again.
You can still use snapshot by creating them while tour server is up, but don't forget to delete them before reboot.
Waiting for a grub2 upgrade...