If you wish to install the new Solaris 10 10/08 using ZFS for its root file system, you might not see it in GUI installation mode.
That’s why you need to install it using Text/Console mode. Just choose number 4 in the first prompt of the installation process.
In one of wizard step, you are prompted either using ZFS or UFS, just choose ZFS instead. Then, fill the pool name, choose the size of root pool swap and dump area, and how you want to store /var.