I agree that just the decline in blog posts by itself doesn't mean that much.
But combine that with the fact that *nothing* from Moller/Freedom Motors has mentioned David since July (recall that for the past two years, he has largely been the public face of the company), and it strongly suggests that he has reduced his involvement.
Two years ago, I predicted that David's "arc" with Moller/Freedom Motors would match that of many others (Zack, Ed De Reyes, even Jeff!) -- Initial enthusiasm, followed by gradual disillusionment as the nature of the company came clear, and ending with departure. We seem to have witnessed that arc over the past two years.
