Recently Oracle integrated persistent L2ARC in ZFS and this is currently available in the ZFS-SA. It is not yet in Solaris 11 but it should be coming soon. Finally!
To make the very good news even better - it stores blocks in their raw format, so if for example you have compression enabled in your pool then L2ARC will store blocks compressed as well (similarly for encryption). If your data compresses well you L2ARC suddenly became much bigger as well.