A new micro update to the 2008.Q4 (formerly "1.0.x") software has been posted. This includes a fix that addresses incomplete resilvering when replacing disks within a RAID stripe. All users with single- or double-parity RAID storage pools that have not yet switched to the 2009.Q2 release are strongly encouraged to upgrade to this release as soon as possible. Data is properly resilvered during a scrub, so you can also scrub your storage pool if you have replaced a disk.

Complete release notes can be found here.