As far as i know, it hasn't been 100% verified but somewhat hinted at and 'sorta' verified by some unreliable sources.
Please correct me if im wrong.
Thanks.

Edit: Ok. I get it. Most of you believe the topic has already been verified by magazines. ( I'm set on the line until Todd in a video interview actually says.. ' Yes. The Dark Brotherhood will be in Skyrim. ' )
Anyways. That doesn't change that you can still vote on it as it is clearly listed as " Do you think the Dark Brotherhood SHOULD be in Skyrim ". Not WILL it. SHOULD it. That means that even if it is, it is a matter of do you think that it belongs in it.
Thank you to those who can read. :thumbsup: