When Motorola began locking their device’s bootloaders, we were a little nervous and hoped no one would follow; well it seems we were wrong. The device we have been impatiently waiting for has finally made its way to the market, but the latest news from Androidpolice is that the device is locked. The Android blogs source says the device does not come with the usual customizability that HTC devices are known for, but actually is locked down like the G2 was at launch. The bootloader does not act as a normal HTC bootloader would, it always checks the ROM you have installed on it at boot and if it’s not signed by HTC, it will not allow you to enter recovering.
This seems like something that might add to the reasons why you guys might not be interested in acquiring this HTC made device anymore, but we’ll let you decide for yourself.
How it works (Explained by AndroidPolice):
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.
Source: HTC Thunderbolt Takes A Page From Moto’s Playbook: Bootloader Is Locked | Android News, Reviews, Apps, Games, Phones, Tablets, Tips, Mods, Videos, Tutorials - Android Police