Conspiracy theorists would have you believe that the ThunderBolt's signed (and locked) bootloader was all Verizon's doing, but it appears that isn't quite the case - the Incredible S, one of HTC's unlocked GSM phones, is shipping with a similar failsafe system. That basically means no custom ROMs for you (at least until a viable workaround is discovered).
Proof? Look no further than the contents of this Incredible S RUU:
From what our friends at AndIRC can tell (note that they don't have a device in hand), the Incredible S includes a signature check much like the one on the ThunderBolt. It kicks in if you somehow manage to flash a custom recovery image to your phone and will then proceed to examine the "signature" of said recovery image. If it's anything other than the stock HTC key, you won't be allowed to access recovery. Just as with the ThunderBolt, that means you won't be able to flash custom ROMs - exactly why HTC designed this system.
But why has HTC, previously the most dev-friendly manufacturer of them all, suddenly turned against the modders? We asked the company this exact question at CTIA, only to be told that they are just catering to carriers' interests. We have a hard time believing this, however, as the Incredible S is, again, an unlocked GSM device - therefore, it couldn't have been the carriers' fault. If you ask me, they're simply trying to decrease the number of users who pester customer support with devices that are bricked as a result of unsuccessful system modding. Or, of course, Peter Chou's long-lost evil twin could have seized power at HTC (which might also explain the firm's recent uncharacteristic lack of innovation).
Whatever the reason behind these actions may be, this probably marks the end of the dev-friendly HTC we've all come to know and love, and that breaks my geeky heart. Farewell, HTC. You will be missed.
Thanks to scotty2 for pointing this out!