If so, be patient. In fact it is a high-wanted feature, and the ZFS team is working hard on it right now. You can learn more about this feature following the Shrinking a zpool? thread on the zfs-discuss forum on opensolaris.org. Here are some chosen excerpts.

From Matthew A. Ahrens #1:

Regardless of where you want or don't want to use shrink, we are actively working on this, targeting delivery in s10u5.

From Matthew A. Ahrens #2:

Yeah, the implementation is nontrivial. Of course, this won't have any impact on snapshots, clones, etc. and will happen on-line. Any other solution would be unacceptable.

Howdy... I really like these kind of short and concise answers!