Apple's first attempt at rolling out support for its new Thunderbolt devices ended in tears when it stopped many Macs being able to boot properly. Fortunately, the problem's now fixed.
The new update, which provides support for Apple's new Thunderbolt to Gigabit Ethernet adaptor, doesn't seem to mess up Mac OS X. That means that if you did the sensible thing and put off updating until the problem was fixed, you're now good to go. The update is available now. [9to5mac]