Made this one with a 'city' and a flying jet, both created in Hexagon. That was just half an hour work. I created the clouds in Bryce, based on a tutorial by David Brinnen, using an infinite cloud plane with a volumetric material. That took me many hours to get something I liked. I started with another basic setting than David (waves instead of fractal). Unfortunately, I could not get rid of too much banding in the closest clouds. For lighting I used HDRI created from the sky, together with the sun. I textured everything in Bryce and rendered with premium settings, with true ambience (excluding the clouds, otherwise it would be rendering for over a week, I think). Then I exported to HDRI and did a little postwork (tonemapping) in Picturenaut, making the picture a little more purple and more 'crisp'. To limit the banding effect, I added a small Gaussian blur to the two closest clouds (in Photoshop Essentials). You see, quite a complicated work-flow, but I think it was worth it. I would really appreciate some feedback: what do you like in this render (if anything) and what doesn't work in your eyes?