From his blog-

Last week I decided that I would leave my contract position with Wizards of the Coast. I am no longer working on Dungeons & Dragons, although I may provide occasional consultation in the future. 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. I enjoyed every moment of working with them over the past year. I have faith that they'll create a fun game. I'm rooting for them.
There is more at the link.

Thaumaturge.