Update: If you've somehow ended up on this page, please note: HTC has since changed its bootloader policy - all future phones will be unlocked.
Once a great example of how open Android phone manufacturers could be, HTC has taken a lot of heat from its fan base over recent decisions to lock its devices' bootloaders (such as with the Sensation and EVO 3D), endangering the possibility that these devices will be able to support custom ROMs. Fortunately, there is a chance that could be changing, as HTC announced on its Facebook page that the company is reviewing its bootloader policy:
Thanks so much for providing feedback, we hear your concerns. Your satisfaction is a top priority for us and we're working hard to ensure you have great experiences with our phones. We're reviewing the issue and our policy around bootloaders and will provide more information soon. Thank you for your interest, support and willingness to share your feedback.
Remember, this should definitely be taken with a grain of salt - just because HTC is reviewing the current policy doesn't mean it has any plans to change it. And if it does change, there is always the possibility of those pesky carriers getting in the way.
Source: HTC Facebook