Following this morning's teaser from Bungie, Xbox's Major Nelson has confirmed the date of the Halo 3 Mythic Map Pack.
Straight from the Major:
Halo 3: The Halo 3 Mythic map pack, currently only available to those that purchased the Limited Edition of Halo Wars, will be available to everyone on April 9th for 800 points.
Gears of War 2: On March 31st the Snowblind map pack will be available for 800 points. The map pack contains four new Winter themed maps: Grindyard, Under Hill, Courtyard, and a frozen version of the "Gears of War" favorite, Fuel depot. In addition, a title update is on it's way that contains various improvements and fixes as outlined here.