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Support for reading it must be done in bootloader (GRUB for Solaris x86). So one have reimplement part of filesystem, in pretty hostile environment (bootloader is simple compared with fullblown OS with memory management and stuff). Also volume management part of ZFS makes it harder -- you can't just read some sectors of hard drive, you have to recreate stripes, mirrors, raids first.
But it's doable. Solaris people were working on it ( http://opensolaris.org/os/project/zfsboot/ ) and it's now possible to boot Solaris from ZFS on x86 ( http://www.opensolaris.org/os/community/on/flag-days/pages/20070328... ).