The new Thunderbolt firmware version 1.2 comes over five months after Apple released v1.1, which fixed an issue causing some MacBook Pros to see malfunctions when using Thunderbolt cables with a bus-powered device.
From the release notes:
About Thunderbolt Firmware Update v1.2
This update provides stability fixes for Thunderbolt and Target Disk Mode.
When your computer restarts a gray screen will appear with a status bar to indicate the progress of the update.
Do not disturb or shut off the power on your computer during this update. Note: Your computer must be plugged into AC power for the update to install.
The latest Thunderbolt firmware comes in at 1.22MB and requires OS X Lion 10.8.3 or later to run. Users can download the update via Software Update or Apple's Support website.