OpenBSD does this by default. It also now directly boots cryptodisks eliminating the need to create a /boot partition and carry it around if you're concerned about evil maid attacks, though I would imagine a camera or keyboard hardware keyloggers would defeat that pretty easily

The boot loader is still on the disk, unencrypted.

That's what removable media is for

derp, /root correction

As does OSX. Probably Windows too? There has to be a registry setting to tweak, somewhere.