http://openphotovr.org uses many CC-licensed images from Flickr, honoring license terms and giving attribution with backlinks. For example, we built San Marco from Flickr, unlike Photosynth's San Marco which was shot in one day by one photographer.

The problem with architectural photos on Flickr: everybody chooses the "best" viewpoints. This is why I couldn't build a model of Taj Mahal, one of the most photographed places on Earth - all good images of it are taken from the same point.