Apple has, with no change in the release notes or acknowledgement of the problems caused by version 1.2, released Thunderbolt Software Update 1.2.1 to add support for the Thunderbolt to Gigabit Ethernet Adapter. Version 1.2 of the update was notable for causing boot failures, and although the assumption is that Apple addressed the problems, we won’t know for sure until reports start to come in from users.
If you installed version 1.2, then this new update will not show up in Software Update. It can be downloaded here, but there’s no need to run this on top of the old update.