From Art Geek:
Until today, I was frustrated by a minor “feature” in SketchUp that no matter how one wheeled about one’s model with the Orbit tool, the horizon was always horizontal. That meant exporting larger 2D images and then rotating and cropping in Photoshop or iPhoto.
Today, I accidentally discovered that holding down the 'option’ key (on a Mac) while orbiting allows one to tilt the horizon in the camera’s view. A minor trick, perhaps, but one I had wanted, and all the more enjoyable for its accidental discovery.