There's a huge difference between just switching your desktop and switching your entire distribution, but it's still easy. Just plow under the OS partition with something else.
And this is actually one of the great things about zfs: you can now have a volume manager and filsystem that is fully portable across FreeBSD, (Open)Solaris and Linux. Keep /home on a zfs volume, and you can really mix things up.
The only thing missing is solid cross os encryption - but for those that can live with some metadata being exposed, it looks like encryption is coming [ed: to open zfs].
I suppose one might even share home filsystems with OS X - but running OS X requires Apple hardware (and/or breaking the license and some hacks).
[Ed: and sharing home filsystems across unix-like os' actually works: thanks to things like the Bourne shell and other software being available and using the same storage for preferences etc (typically text files) - Bsd even comes with an emulation layer to run Linux executables.]
