I am particularly interested if there are any network mounted disk types that can be used in ZFS.
ZFS can use any block dev that you present to it, so you're really just bound by what your OS and environment support. Since you're asking about Solaris, this means that FibreChannel (FC), iSCSI (as mentioned above by Rowley) and any other block-device technology available in your environment (InifiBand?).
ZFS can also use files (on top of UFS, or a ramdisk, or whatever else you can dream up) as data sources, which is very powerful for testing vdev configurations as you're getting used to the technology and tools.