I would argue the "significant landmark" problem has essentially been solved as a side effect of online photo sharing sites (originally Flickr, now everything) -- the most frequently photographed things are the landmarks, and the number of photos scales with the significance of the landmark. When you search for "Rome" on Flickr, the clusters of photos that pop out as being 3D-reconstructable are precisely the landmarks.
Quoting:
"The data set consists of 150,000 images from Flickr.com associated with the tags "Rome" or "Roma". Matching and reconstruction took a total of 21 hours on a cluster with 496 compute cores. Upon matching, the images organized themselves into a number of groups corresponding to the major landmarks in the city of Rome. Amongst these clusters can be found the Colosseum, St. Peter's Basilica, Trevi Fountain and the Pantheon."
See this popular work from 2009, "Building Rome in a Day": http://grail.cs.washington.edu/rome/
Quoting: "The data set consists of 150,000 images from Flickr.com associated with the tags "Rome" or "Roma". Matching and reconstruction took a total of 21 hours on a cluster with 496 compute cores. Upon matching, the images organized themselves into a number of groups corresponding to the major landmarks in the city of Rome. Amongst these clusters can be found the Colosseum, St. Peter's Basilica, Trevi Fountain and the Pantheon."