Announced at WWDC, and the only way to get Firewire peripherals working with the new Retina Display MacBook Pros and MacBook Airs (besides a Thunderbolt Display), the new Thunderbolt to Firewire cable is now available at the Apple Store online. (Thanks, SchulMarm!)

The adapter ships in one to three business days and is $29.

(Update: the page appears to be off and on)