Frictional Games seems to be working on a follow-up to Amnesia: The Dark Descent, if a teaser site is any indication. For the time being, we know nothing about the game except that it carries the "Amnesia" name, and has some kind of connection to China.
The site NextFrictionalGame.com (via Rock, Paper Shotgun) simply carries the words "Amnesia" and "Something is emerging." Clicking on the title image takes you to a Google Map of China. Checking a whois of the site confirms that it's from frictional, and the meta keywords include phrases like "horror," "scary," and "work in progress." The description states that right now "it does not have a final name decided."
It's not much to go on, but it is something. The first Amnesia went on to critical acclaim, winning Independent Games Festival awards for audio, technical excellence, and the Direct2Drive Vision Award.