Patch 19 introduced a few new bugs including a problem linking back to test instances from defects. This has now been fixed (patch 24) but the next formal public release is supposed to be patch 25, which was due for release on 15/06/2011. Not sure if this has happened yet as we are waiting confirmation.
On a side note (and for Saas users) the Tools (Site admin mini) functionality is due to have an update in July 2011.
Does anyone have a link to the release notes for patches?