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.