Mike Mearls (Senior Manager, Dungeons & Dragons Research and Design) has updated the ravening masses via this week’s Legends & Lore column that the 5th Edition Dungeon Master’s Guide, orginally scheduled for November 19th, has been delayed:
“The new release date for the Dungeon Master’s Guide is December 9th. For those stores that take part in our early release program, the book goes on sale November 28th.”
The long of it is that extra editorial effort on the Player’s Handbook (already out as of August 19) and the Monster Manual (releasing September 30) was accomplished at the expense of prep time for the DMG.