Although there has yet to be word from Dahlberg himself, Joanne Laroche, the Senior Community Coordinator, recently posted this on the official Old Republic website: "Sean Dahlberg has recently left BioWare. We would like to thank Sean for doing a great job building the TOR Community and we wish him well in his future endeavors!" This confirms that Dahlberg has left BioWare, but it still does not answer why this has happened.
Massively will keep an eye on this story and on Dahlberg's personal site for new developments. Until then, we wish the best for Sean and for the game.
[Nice catch, "fo diddy" at TOROcast!]
Update: Sean posted on his personal blog and wanted to let everyone know that he did not fall off the end of the earth. As for what's next for Mr. Dahlberg, he had this to say, "What's in the next chapter in the life of Sean... well, that would be giving out information and we all know I don't do that. I'd say just kidding but I actually can't talk too much about that at this point. Soon™ but not yet."