Howtoforge published a tutorial about setting up software RAID1 on a running LVM system under Ubuntu 11.10

This guide explains how to set up software RAID1 on an already running LVM system (Ubuntu 11.10). The GRUB2 bootloader will be configured in such a way that the system will still be able to boot if one of the hard drives fails (no matter which one).

  How To Set Up Software RAID1 On A Running LVM System (Incl. GRUB2 Configuration) (Ubuntu 11.10)