So, being more than satisfied with the DP2M, I decided, having seen the cashback offer from Sigma UK, to get the DP1M - in fact, since the DP1M arrived, the DP2M has sat on the shelf, since the wider angle fits the way I view the world much better than its narrower-angle sibling. But to each his own, and I'm sure that the DP2M will get plenty of use.
So there I was, taking a few snaps with my new camera, and I turned it 90 degrees to get a vertical shot. And the screen went blank. I'd left the OVF at home that day, to keep things compact, so I had no alternative means of framing. So I tried again - horizontal OK, vertical no image. So I went through the menu to see if I'd missed anything - no luck there. But then something occured to me. I took off my Polaroid sunglasses. And everything worked perfectly.