The long-awaited Thunderbolt to FireWire Adapter made its debut on Apple's online store this morning, but the product page appears to be appearing and disappearing at will. I wasn't able to see the cable listing, but Erica Sadun and Dave Caolo did; as of 11am ET, it appears to be down again. Assuming it does go on sale today, the cable will cost $29.99 and apparently will ship in one to three business days.
A Thunderbolt update was pushed out in June to provide compatibility with another new Thunderbolt goodie
