PhotoSketch by Brainstorm Technology LLC is now available with a fully functional 30 day trial version available for download. PhotoSketch is a Google SketchUp plugin that enables the user to model 3D urban structures directly from photographs. It is aimed at architects, designers, and entertainment content creators allowing a new approach to create lightweight 3D models of existing buildings.
Sure you can model direct onto photographs already in SketchUp and indeed that is the basis of a lot of the models currently seen in Google Earth. Where Photoshetch seems to differ is the ability to determine camera positions and extract and match geometric features derived from a collection of overlapping photos of an urban scene.
Head over to http://www.brainstormllc.com/ for the download.