Monte Cook just posted to his Live Journal Blog that he "decided that I would leave my contract position with Wizards of the Coast. I am no longer working on Dungeons & Dragons".
He also states, "My decision is one based on differences of opinion with the company. However, I want to take this time to stress that my differences were not with my fellow designers, Rob Schwalb and Bruce Cordell."
Anyone can see that this isn't a good sign. Because it is an disagreement with "the company" my mind immediately went to issues with the gaming license. There is no strong evidence for that, but I know that Monte worked hard on the OGL with D&D 3.x. Given that Hasbro seems to not have cared much for the OGL and reversed course so much with 4E I can't help but think that Hasbro is only willing to sacrifice D&D as a whole rather than just make a great game with a great community to support it.
Because of the NDA that Monte had to sign to even work on the project, we may never really know why. If by some miracle, all the lawyers and corporate types are Jedi mind tricked into making D&D open again, I will always believe to my core that Monte left because of that.
Good luck Monte! DNDNext will miss you.