What is happening:
When players glitch and fall beneath the Heartlands, there is no kill plane that respawns the player. Instead, the player falls onto an invisible surface below the ground, able to walk freely across the entire Heartlands map.
It is far enough below that unicorns are not able to teleport back up, and logging out and back in does not automatically return the player to the surface. The only way to get out of this situation is to use the /stuck command. (Which, if I am right, will be removed upon release.)
Aside from the teleportation glitch (near the burnt-out area in the Heartlands) that got me under here, a kind of glitch that has been reported before, there should always be contingency plans for future glitches that may not be caught. Therefore, there should be something underneath the Heartlands that is similar in function to the plane below Cloudsdale.
This was found on version 0.1.4, on a Linux 64-bit system.