Apple has released an update to all Macs with Thunderbolt ports, adding support for the new Thunderbolt to Gigabit Ethernet Adapter. The update weighs in at 520KB and requires OS X Lion 10.7.4. It can be downloaded from Apple's software update page.
About Thunderbolt Software Update 1.2.1
Adds support for the Apple Thunderbolt to Gigabit Ethernet Adapter.
The update is a replacement for Thunderbolt Software Update 1.2, which was released last week but was pulled by Apple after users reported boot failures upon installing the update. Apple has posted a support article addressing the issue with instructions on how to resolve the boot problems.