That didn't take long. A collaborative effort between numerous Android hackers just managed to crack the Thunderbolt's bootloader wide open, successfully booting a custom recovery image. They also incidentally discovered a new root method in the process. Credit goes to jcase, jamezelle, and scotty2 (and all of andirc) for working out the details, and ProTekk and Trident for putting their shiny new Thunderbolts on the line. Here's the visual proof:
Image credit: Trident, ProTekk
We'll keep you apprised on the situation with instructions on how to go about unlocking your own Thunderbolt as we get more information.