I was told by a birdie that both Preorders and Blackboxes should start/hit stores next week. This information is in line with the prophecy.
Grey Knight preorders will be up next week. Black boxes with Terminators, Dreadknights and the codex are headed to the Games Workshops too.
More information puts the dates set at March 9th for the Black Boxes to arrive, and March 25th for Grey Knights to be available for Preview. Check here for more information and sources http://the-grey-knights.blogspot.com/2011/03/rumors-some-dates-for-your-calendar.html