Belfiore says 8.1 going out in Dev Preview the first part of April.

Just saw this hit twitter.  Still no exact date for when this will happen but keep your eyes peeled between now and then.  I’ve heard as early as the 10th or as late as the 14th by this estimation of when things will be sent out.  To make sure you’re prepared for the update when it gets sent out head over to my other post on how to sign up for an App Studio account so you get the OS update for free and bypass all these carrier shenanigans.

Lots of questions from everyone… Here's SOME answers. Timing for dev preview… "first part of April". We don't have exact date yet.

— joebelfiore (@joebelfiore) April 6, 2014