I see that many people are asking about ZFS + Sun Cluster solution. Soon Sun Cluster 3.2 should be released which does support ZFS (among many other new features). Now Solaris 10 is free, Sun Cluster is free. Additionally to install Sun Cluster it's just some clicks in GUI installer and voila! Then some other commands and we have ZFS pool under Sun Cluster management.
Below example (using new SC32 commands, old one are also available for backward compatibility) how to configure 2-node HA-NFS cluster with ZFS - as you can see it's really quick&easy.
Nodes: nfs-1 nfs-2
ZFS pool: files
# clresourcegroup create -n nfs-1,nfs-2 -p Pathprefix=/files/conf/ nfs-files
# clreslogicalhostname create -g nfs-files -h nfs-1 nfs-files-net
# clresourcetype register SUNW.HAStoragePlus
# clresource create -g nfs-files -t SUNW.HAStoragePlus -x Zpools=files nfs-files-hastp
# clresourcegroup online -e -m -M nfs-files
# mkdir /files/conf/SUNW.nfs
# vi /files/conf/SUNW.nfs/dfstab.nfs-files-shares
[put nfs shares here related to pool files]
# clresourcetype register SUNW.nfs
# clresource create -g nfs-files -t SUNW.nfs -p Resource_dependencies=nfs-files-hastp nfs-files-shares
ps. right now it's available as Sun Cluster 3.2 beta - I have already two SC32 beta clusters running with ZFS and must say it just works - there were so minor problems at the beginning but developers from Sun Cluster team helped so fast that I'm still impressed - thank you guys! Right now it works perfectly.