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.

ZFS boot support didn't really have a bearing on scaling up large FreeBSD storage servers. It is standard practice for various reasons to have a seperate RAID (or SSD) for booting large fileservers.

OpenSolaris had so many restrictions on the boot ZFS pool (could only be a single disk or RAID1 mirrored) anyway.

It is ironic also that you mention driver availability, when driver support is one of the problems with FreeBSD. Commercial support has been quietly but steadily eroding for the last few years. If you try to put together a big FreeBSD system, as I have done, you start to run into trouble once you leave the lower levels of consumer-grade hardware.

If you want a viable alternative to OpenSolaris, look at Nexenta. They have recently hired some of the people who left Sun when Oracle took over.

I particularly liked that it has PPC G5 SMP support as well, as then I may be able to get some use out of the retired G5s as ZFS file servers.

Edit: On the other hand, it uses quite a lot of power that thing. So maybe not...