HTC has finally laid out a timetable for the release of software updates on phones that will allow the unlocking of device bootloaders. It sounds like HTC will be utilizing a system similar to Sony - which uses a web-based tool as part of the unlock process. Why? HTC states that while the OTA software update allowing unlocking will start rolling out in August, the actually ability to unlock phones won't be ready until early September. So, at the earliest, your EVO 3D or Sensation 4G will be unlockable in about 2 months.
Unfortunately, there's another caveat: HTC hasn't exactly made it 100% clear that August is the date for the release of said software update for the EVO 3D and Sensation 4G.