FreeBSD 9 - default file system?
Solution 1:
It's still UFS. The installer (bsdinstall as of 9.0) cannot install to a ZFS filesystem.
If you want ZFS for your root filesystem you need to do a fully manual install.
Solution 2:
PC-BSD installer can install FreeBSD on ZFS. As soon as the installation start, you may choose you want to install FreeBSD (instead of PC-BSD) and then the installer lets you partition your disk using ZFS.