Apple released Thunderbolt software update version 1.2 yesterday (which has now been pulled) with the lone change of "Adds support for the Apple Thunderbolt to Gigabit Ethernet Adapter."
After the update, hundreds of users fled to the Apple support forums . A handful of users after the update have not been able to boot properly. There have been reports of odd Kernel panics, or simply just getting stuck at the boot menu.
The only solution for now seems to be a boot into recovery and reinstalling Lion. For now, we warn you to stay away from this update until Apple posts a fix. Since the update is already pulled, hopefully a fix will follow soon.