Because I'm concerned about the future of OpenSolaris, I downloaded 8.1 and attempted to get a ZFS boot install going -- I was unsuccessful. The process is extremely complex, and the best doc you're going to get is a wiki page with incomplete and complex instructions:
http://wiki.freebsd.org/RootOnZFS/GPTZFSBoot
Moreover, there's no support for ZFS boot in the installer (sidebar: the installer has barely changed since when I first tried FreeBSD 2.2 in 1998).
I'd say stick to OpenSolaris if you require ZFS boot; alternatively keep boot on UFS and use ZFS for your data filesystems.