He's still a producer on the show, and will return on the 200th episode, but he is no longer a regular on the show. I think he wanted to spend more time with his family.
I have a question - everytime I read about RDA's desire to leave Stargate SG-1, it says he wants to spend more time with his daughter. His wife isn't mentioned. Is he divorced? (Not that it matters to me, of course, being a happily married woman; just curious.)
For season 10 RDA will be in 5 episodes to tie up some plot lines with his character. If they get him and Carter together they better not make it too soapy, although chances are he will be killed soon after they start a relationship. :)