Update: Bootloader and recovery are both locked and signed. See new screenshot.
Update #2: The device has been rooted, and its bootloader has been unlocked!
Some of our good friends over at AndIRC (most thanks going to scotty2 for discovering it, and Trident for being the guinea pig) have discovered a bit of troubling information regarding the HTC Thunderbolt: its bootloader and recovery are locked. What does that mean, exactly? No custom ROMs - for now. Some enterprising hackers successfully flashed a custom recovery image onto a T-bolt, but lo and behold, upon entering the bootloader, no recovery boot option was to be found once the flash had been completed.
Similar security measures have been put in place on Motorola phones. The basic gist is this: if you somehow manage to flash a custom recovery image to your phone, a failsafe (known as a signature check) will kick in on boot-up and examine the "signature" of the recovery image loaded onto your phone. If the known "good" signature (HTC's key, basically) stored on the phone's read-only-memory doesn't match the signature of the recovery image, the phone's bootloader won't allow you to access recovery.
Image credit: Trident
This, in turn, means you can't go flashing ZIPs and custom ROMs to your phone willy-nilly, which is what the manufacturers want to keep you from doing. Of course, it's only a matter of time until a way around this obstacle is discovered - and we have it on good authority that scotty2 and the AndIRC team have a surprise in store for us very soon. So, hold onto your butts.
AndIRC, scotty2, Trident