I knew there was a reason to understand the ZFS on-disk format besides wanting to be able to teach about it...
Recently, I was sent an email from someone who had 15 years of video and music stored in a 10TB ZFS pool that, after a power failure, became defective. He unfortunately did not have a backup. He was using ZFS version 6 on FreeBSD 7, and he asked if I could help. He also said that he had spoken with various people, including engineers within Sun, and was told that basically, it was probably not possible to recover the data.
He also got in touch with a data recovery company who told him they would assign a technician to examine the problem at a cost of $15,000/month. And if they could not restore his data, he would only have to pay 1/2 of the cost.
After spending about 1 week examining the data on the disk, I was able to restore basically all of it. The recovery was done on OpenSolaris 0609 (build 111b). After the recovery, he was able to view his data on the FreeBSD system (as well as OpenSolaris). I would be happy to do this for anyone else who runs into a problem where they can no longer access their ZFS pool, especially at $15k/month. And if I can not do it, you would not need to pay anything!