This is a great thread!

John, perhaps your grumpy spell was just what was needed to bring out all of these great, thoughtful replies.

Leonore, I think you may be onto something regarding the familial PD being a somewhat different animal with respect to depression. Several years before he was diagnosed with PD, I witnessed my older brother's struggle with depression, and in retrospect, now realise that was probably what led to his early retirement as a school principal.
My own depression preceded my Dx by some 10 years. In spite of trying several antidepressants and undergoing psychotherapy during that time, it never fully lifted. I continued to perform pretty well professionally until about two months before the PD Dx, when I became unable to prepare new graduate lecture material. I took medical leave, and finally realised that I was dragging my left foot slightly, and was experiencing some postural instability and dizzy spells. After numerous inner ear tests by Ear, Nose and Throat specialists, MRIs by one neurologist looking for evidence of stroke lesions, with little to show for it, I saw the head of our Neurology department, who watched me walk up and down the hallway, then said, "I think you may have Parkinson's disease." The positive response of both the foot drag and the depression to Sinemet removed any doubts of the Dx. I found that Sinemet had greater antidepressant effectiveness than anything else I had previously taken. I was able to return to work within a couple of months, completing my career before retiring at 68 in 2006.
In my case, the profound antidepressant effect by Sinemet was likely due to relief of both the motor symptoms as well as mental anxiety resulting from the motor symptoms. In all of that time, I experienced little if any loss of positive self image.

Robert