HTC EVO 4G OTA update 4.67.651.3 unlockable at HTCdev
While not listed on the drop-down menu at HTCdev, it seems that the newest OTA update for the HTC EVO 4G, which is currently unrootable via Revolutionary or UnrEVOked, is officially unlockable by using the "All Other Supported Models" option.
This unlock method is not the same as S-OFF. It gives users a bootloader that allows flashing, but radio images cannot be flashed. So if you've lost root and are attempting to go back to a normal flashing life, you may need to learn a couple of new steps to flash kernels, and you're simply not going to be able to flash radios.
While unlocking via HTCDev can theoretically void your warranty, at the moment if you're stuck with version 4.67.651.3/HBOOT 2.16.0001+, you'll need to unlock the bootloader officially or wait to see if any developers can crack HTC's newest HBOOTs (it's not looking so promising lately, by the way).
After unlocking, you'll then need to flash a recovery and use it to install either superuser or a ROM.[HTCdev] Thanks, Captain_Throwback!