Evo 3D to have locked down bootloader

Bad news for the root community: it looks as though HTC is taking the same
lock-down approach as Motorola with the bootloader check on their new HTC EVO 3D. This means that root access, custom ROMs and custom kernels may never make it to the upcoming Android phone for Sprint.

Normally,
when your Android device boots up, it will run a check of the
bootloader, kernel, and recovery images to make sure each has the
correct signature (authentication key). With a regular signature, it is
fairly easy to sign a custom ROM or kernel and trick the device into
accepting the custom image. These signatures can be copied from a full
RUU (ROM Upgrade Utility) if one is available for the device. 

Unfortunately, HTC is taking a page from Motorola and using cryptographically signed
images, which use 256 bit encryption to protect the signatures. This
was confirmed by Justin Case of Android Police. Essentially, this means
that there is almost no chance of a crack coming through. We say
"almost" no chance because there have been cryptographic signatures that
have been cracked before, most notably the Sony PS3.

This same security can be found on the HTC Sensation, Motorola DROID 2 and DROID X, so don't expect much in the way of custom ROMs, kernels or recovery on those devices either.

source: Android Police