Update: This method is outdated - please refer to this guide instead.
Jamezelle, jcase (that's me), and all of AndIRC (i.e. everyone - AndIRC is an open dev group), the same folks who rooted the pre-production Thunderbolt in February, proudly present:
A Very Dirty HTC Thunderbolt Root V1.02 2011/03/18
- Root with read/write access to /system
- Ability to downgrade and flash any RUU (i.e. signed firmware)
- No custom recovery [yet]
- No custom kernels [yet]
- The root procedure currently requires flashing a slightly older version of the firmware (RUU_Mecha_VERIZON_WWE_1.05.605.0_Radio_1.07.00.0108r_NV_8K_1.38_9K_1.54_release_166255), which could potentially have more bugs.