Full disk encryption without a boot partition

[mhca12]

On Thu, Dec 27, 2012 at 10:08 PM, Alex Hornung <alex at alexhornung.com> wrote:
> On 26/12/12 22:19, mhca12 wrote:
>> Are there any plans or is there already support for full
>> disk encryption without the need for a boot partition?
>
> No, the userland tool that sets up the decryption of the root partition,
> as well as the kernel and modules need to be somewhere that is not
> encrypted - otherwise the boot loader would need to support the disk
> encryption.

Seems like OpenBSD 5.2's bootloader can do that.
Any idea how they did it?