HBO released a very, very short promo for season 2 as part of a special upcoming projects feature before the season premiere of Boardwalk Empire. Here it is:
Not having witnessed the full promo myself, I did see several different people indicate April 2012 for the premiere following the original airing of this piece on HBO. Was there something in the full teaser shown on HBO that made that clearer? I can’t say (if you can, please clarify in comments!), but in any case, it does seem a rather safe bet as HBO really doesn’t have that much room in which to place the show.
As to the voice over? We’ve been informed that that’s the voice of Stephen Dillane, aka Stannis Baratheon.