Migrating rpool to smaller drive
There is a very good tutorial at:
http://virtuallyhyper.com/2012/08/migrating-the-root-zfs-pool-to-a-smaller-drive/
Just one addition: After creating the Slices (e.g. changing slice 0) don’t forget to run ‘label’ from the ‘format’ utility, otherwise your changes won’t be written!
You can find a PDF of the blog entry: Migrating the Root ZFS Pool to a Smaller Drive – VirtuallyHyper