Howtoforge posted a tutorial about converting an ext3/4 file system to btrfs on Ubuntu 12.10

ext3 and ext4 file systems can be converted to btrfs. For non-root file systems, this can be done online (i.e., without reboot), while for root file systems we need to boot into some kind of rescue system or Live CD. This guide explains how to convert an ext3 or ext4 root file system into btrfs on Ubuntu 12.10 and how to roll back to ext3/ext4 again if desired.

  How To Convert An ext3/ext4 Root File System To btrfs On Ubuntu 12.10