The main reason we're wasting our time with all this is ZFS. In my case, I'm building a nice 16x1T backup machine and want to utilize the disk space as efficiently as possible. This is a bit of a challenge, since installer is just a braindead shell script and offers you only two options: install on zfs root mirror with two whole disks or manually partition and install on one disk only.
Without previous ZFS expirience one chooses zfs root mirror and then wastes some days trying to figure out how to shrink the pool and retain the working grub configuration. I gave up as soon as I realized that I can attach another block device to an existing zfs pool and get a mirror automagically :)
So the answer is to install manually on one disk, where you create a small (eg 20gb) root slice, 1gb swap slice and the rest for /export/home or whatever. Let the install finish, reboot, format the other drive that will become a mirror to the one you installed on (warning, solaris slices! :) and then use zpool attach <poolname> <existing device> <new device> to create a mirror. What remains is just installgrub on the new device and voila, you have a mirrored bootable zfs root gnu/opensolaris system.
Few tricks: if format complains about not being able to relabel a disk that is in use, simply export NOINUSE_CHECK=1. Also when creating a zpool from slices, it complains that s0 overlaps with s2 (which should be that way, if I understand slices corectly), but we can simply use zpool create -f. After you created your zpool and zfs volumes to your desire, be sure to check zfs get all <name> and check what all can you set on them. I found compression=on and atime=off very userful. Also set snapdir=visible if you plan to offer backup history via shapshots. It's amazingly simple :)